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Biomass to hydrogen via catalytic steam reforming of bio-oil over Ni-supported alumina catalysts
Fuel Processing Technology 92 (2011) 563–569
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Biomass to hydrogen via catalytic steam reforming of bio-oil over Ni-supported alumina catalysts F. Seyedeyn-Azad, E. Salehi, J. Abedi ⁎, T. Harding Chemical and Petroleum Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada
a r t i c l e
i n f o
Article history: Received 25 June 2010 Accepted 17 November 2010 Available online 14 December 2010 Keywords: Hydrogen production Pyrolysis bio-oil Catalytic steam reforming Ni/Al2O3
a b s t r a c t Production of hydrogen (H2) from catalytic steam reforming of bio-oil was investigated in a fixed bed tubular flow reactor over nickel/alumina (Ni/Al2O3) supported catalysts at different conditions. The features of the steam reforming of bio-oil, including the effects of metal content, reaction temperature, WbHSV (defined as the mass flow rate of bio-oil per mass of catalyst) and S/C ratio (the molar ratio of steam to carbon fed) on the hydrogen yield were investigated. Carbon conversion (moles of carbon in the outlet gases to moles of the carbon feed) was also studied, and the outlet gas distributions were obtained. It was revealed that the Al2O3 with 14.1% Ni content gave the highest yield of hydrogen (73%) among the catalysts tested, and the best carbon conversion was 79% under the steam reforming conditions of S/C = 5, WbHSV = 13 1/h and temperature = 950 °C. The H2 yield increased with increasing temperature and decreasing WbHSV; whereas the effect of the S/C ratio was less pronounced. In the S/C ratio range of 1 to 2, the hydrogen yield was slightly increased, but when the S/C ratio was increased further, it did not have an effect on the H2 production yield. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The demand for fossil fuel, considering the available reserves and the pollution caused by continuous consumption, makes hydrogen an attractive substitute energy source [1]. With respect to global issues, hydrogen production from bio-oil produced from fast pyrolysis of biomass is a viable process [2]. Catalytic steam reforming of methane [3,4], propane [5], liquefied natural gas (LNG) [6], ethanol [1], model compounds, such as acetic acid and acetone [7–11], toluene [12], diluted bio-oil with methanol [13] over different catalysts has been widely investigated. A great deal of research has also been carried out on the production of hydrogen by steam reforming of the aqueous fraction of bio-oil [14–19]. By adding water, bio-oil breaks into two immiscible phases: the first phase (aqueous) forms with carbohydrate-derived compounds and light oxygenated compounds; whereas, the second phase (hydrophobic) contains aromatic and lignin derivatives [20]. It is worth noting that the energy yield decreases when the aqueous phase of bio-oil is used, due to the requirement of the primary separation process. Direct use of crude bio-oil has also been studied in sequential cracking processes. Injoiu et al. employed different catalysts, including noble metal catalysts supported on ceria-zirconia, in such a process [20]. Although the high cost of noble metals makes commercial applications questionable, slow deactivation phenomena with time were also observed.
⁎ Corresponding author. Tel.: +1 403 220 5594. E-mail address: [email protected] (J. Abedi). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.11.012
Steam reforming of bio-oil itself was also investigated over the nickel-based catalyst nickel/magnesia (Ni/MgO) in a two-stage fixed bed reactor system [21]. In the first stage, dolomite was used for the primary steam reforming of bio-oil; and, in the second stage, the nickel-based catalyst Ni/MgO was employed for the conversion of methane, in order to increase the purity and yield of the desired product. It was reported that, for efficient conversion of bio-oil to a favored gas product, temperatures greater than 850 °C and S/C ratio greater than 12 were required. The temperatures of the first and the second stages were set at 900 and 850 °C, respectively. The system also had to work at a relatively low GHSV (defined as gas hourly space velocity) to be able to convert methane. Davidian et al. [22] also employed a sequential cracking/reforming process. The H2 concentration in the exit gas stream was 45–50 vol.% over Ni/Al2O3 and nickel-potassium/lithium oxide-alumina (Ni-K/Li2O3-Al2O3) catalysts. Kan et al. [23] obtained a higher H2 yield (87.6%) and tried to reduce the severe deactivation of the catalyst in the production of hydrogen from crude bio-oil. They proposed an efficient approach by using crude bio-oil via the integrative process between gasification and current-enhanced catalytic steam reforming. They used a nickel/copper/zinc/aluminum (NiCuZnAl) reforming catalyst in the downstream process. Domine et al. [24] also investigated steam reforming of bio-oil over monolithic platinum (Pt) and rhodium (Rh)-based catalysts using the steam reforming and sequential cracking processes. They reported that the highest yield of H2 was 70% with the Pt catalyst at an S/C ratio of 10 at 780 °C. According to our knowledge, few studies have been performed on the steam reforming of bio-oil itself in a one-stage process, particularly over
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Ni-supported alumina catalysts. This is partly due to the experimental difficulties incorporated in the testing of a thick bio-oil phase. It is well known that nickel-based catalysts give a high yield of hydrogen for the steam reforming of the model compounds, as well as the aqueous phase of bio-oil [25,26]. Thus, more exploitation of the non-noble metal Ni/Al2O3 catalyst is necessary and significant, where correlative studies on steam reforming of bio-oil itself are very limited. In this paper, we report the investigations performed for hydrogen production over Ni/Al2O3 supported catalysts by steam reforming of the whole bio-oil as a feedstock (not only the aqueous fraction) which is commonly used. The effect of various influential parameters on the hydrogen production is also reported herein. 2. Experimental 2.1. Feedstock and characterization The bio-oil, a dark brown liquid fluid with a strong smoky odor, used in this research was provided by Biomass Technology Group (BTG), The Netherlands. The main elemental composition of the bio-oil was carbon (C), hydrogen (H), nitrogen (N) and oxygen (O) (Table 1); therefore, the oxygenated organic compounds in the bio-oil were generally described by the average chemical formula of CH1.87O0.754, including water. The elemental analysis was performed using a PerkinElmer model 2400 CHN analyzer. The bio-oil also contained impurities, such as sulfur, and traces of metals, such as potassium, magnesium and calcium, which have not been analyzed here. Table 1 also shows the water content, pH, density and higher heating value (HHV) of the bio-oil. The water content of the bio-oil was measured by applying the Karl-Fisher titration method using a Mettler Toledo DL32 Coulometric Titrator. The density of bio-oil was measured with a density measurement bottle, and a calorific value for the bio-oil was also determined using a bomb calorimeter (Parr model 1266). 2.2. Catalyst preparation and characterization γ-Al2O3 and Ni(NO3)2. 6H2O were purchased from Alfa Aesar Company. The γ-Al2O3 used as a support was pretreated at 800 °C for 6 h. It was crushed to a size of 180–250 μm. A known amount of Ni (NO3)2.6H2O was dissolved in water and the pretreated γ-Al2O3 was added to the solution. The pH of the nickel electrolytes and the bio-oil was measured using an Oakton Instruments pH meter calibrated by buffer solutions with pH 2 and 10. The pH of the obtained slurry was about 2. The slurry was continuously stirred at room temperature for 3 h. It was then dried for 48 h in an oven at 120 °C. To make nickel nitrate decompose to nickel oxide, the dried sample was calcined at 700 °C for 3.5 h. Textural properties of the catalysts were obtained by using a Micromeritics Tristar II 3020 instrument. About 100 mg of each of the samples was pretreated overnight at 150 °C. Nitrogen physisorption
experiments were carried out at 77 K to obtain BET surface area, pore volumes and pore size distribution. Temperature-programmed reduction (TPR) of the catalysts was carried out using a Quantachrome CHEMBET 3000 instrument. About 50–80 mg of each sample was placed in a U-shaped reactor. Quartz wool was used to keep the solid sample in place. First, the sample was pretreated under nitrogen (N2) flow (15 ml/min) at 150 °C overnight. The catalyst was then oxidized using air flow (25 ml/min) and heating to 400 °C at a rate of 10 °C/min. The temperature was held at 400 °C for 1 h. Argon (Ar) was then used to flush out any traces of air, while the system was cooled to ambient temperature. TPR analysis was then carried out using 10% H2 in Ar (flow rate 15 ml/min). The catalyst was heated up to 900 °C at a heating rate of 10 °C/min. The signals were recorded using a thermal conductivity detector (TCD). The system was equipped with a cold trap to prevent moisture from entering the detector.
2.3. Apparatus An experimental setup (Fig. 1) was designed and developed to operate at atmospheric pressure and temperatures up to 1000 °C. Two mass flow controllers (MKS, Model M100B) were employed to control and measure N2 (carrier gas) and H2 (reduction gas). A tubular furnace (Thermolyne, Model F21135) was used to provide heat for the system. A tubular reactor with an internal diameter of 1 cm and a height of 43 cm was constructed from stainless steel. A cross shape was welded inside the reactor as a stand for the catalyst bed in the middle of the reactor. Two K-type thermocouples were employed to monitor the temperature of the catalyst bed and the condenser. The operational parameters (temperature, and gas flow rate) were controlled and monitored with LabView software (version 8.5), and recorded data were saved from each run in a data file. The excess steam was recovered by a glass condenser. The coolant was provided by a water-glycol bath circulator (VWR, Model 1150 S). Two high-pressure syringe pumps (ISCO, Model 500D) were used to feed bio-oil and water to the fixed bed catalytic reactor. Water and bio-oil were injected into the reactor through two capillaries that were surrounded by a cooling jacket. This was done to prevent the polymerization of the bio-oil before entering the reactor because bio-oil polymerizes at 80 °C and decomposes upon heating [24]. The needle of the water syringe was located exactly above the needle of the bio-oil syringe. Thus, water droplets entrained the bio-oil droplets on the way to the catalyst surface. After passing through the condenser, effluent gas from the reactor was sent to an on-line micro gas chromatograph (GC) (Varian, Model CP-4900) that was equipped with a TCD. The GC was calibrated by a calibration gas mixture of N2, H2, carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10).
2.4. Procedure Table 1 Characteristics of the BTG bio-oil. Elemental analysis wt (%) Carbon Hydrogen Nitrogen Oxygena Water content wt (%) pH Density (kg/m3) HHV (MJ/kg) a
By difference.
45.83 7.15 0.87 46.15 23.1 2.1 1225.00 17.5
In a typical experiment, 0.2 g of catalyst was loaded into the reactor. For each experiment run, a thin layer of glass wool was placed on the cross-shaped stand to hold the catalyst particles. The electrical furnace was heated to the required temperature. The thermocouple measured the bed temperature every second, and the micro GC took samples of the produced gas, downstream of the condenser, every 4 min. Based on the results obtained from the TPR tests, the catalysts were reduced in situ at 850 °C for 2 h with 400 STP ml/min of a 50 vol.%H2/N2 mixture followed by flashing in nitrogen at 850 °C prior to each run of the experiment. The effect of the Ni content, temperature, WbHSV and the S/C ratio on the yield of H2, CO2, CO and CH4 was investigated in a systematic manner.
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Fig. 1. Schematic diagram of the catalytic steam reforming system: (1), (2) mass flow controllers; (3) bio-oil syringe pump; (4) water syringe pump; (5) reactor equipped with a TC.
2.5. Data analysis Steam reforming of bio-oil can be simplified as the catalytic steam reforming of oxygenated organic compounds (CnHmOk) by the following reaction:
ð
Cn Hm Ok þ ðn kÞH2 O↔nCO þ n þ
Þ
m k H2 2
ð1Þ
The produced gas mixture (CO and H2) is commonly named synthesis gas. CO and the excess steam used in the process further undergo a reaction, named the water-gas shift (WGS) reaction: nCO + nH2 O↔nCO2 + nH2
ð2Þ
Therefore, the overall process can be represented as follows: m Cn Hm Ok + ð2n−kÞH2 O↔nCO2 + 2n + −k H2 2
Different Ni catalysts were compared regarding their activities in terms of H2, CH4, CO and CO2 yields. The hydrogen yield (Eq. (8)) was defined as the ratio of the number of moles of produced hydrogen to the theoretical quantity of hydrogen obtained when complete reforming to CO2 and H2 takes place. Eq. (8) shows that produced hydrogen originates not only from the bio-oil but also from the steam.
ð3Þ
H2 yieldð%Þ =
moles of hydrogen produced × 100 2:181 × moles of carbon fed
ð8Þ
The yield of the other gases (CH4, C2H6, C3H8, C4H10, CO and CO2) was defined (Eq. (9)) as the number of moles of gas obtained per mole of carbon feed. It was obvious that the sum of the CO, C2H6, C3H8, C4H10, CO2 and CH4 yields, called carbon conversion, could not be 100%. The difference related to that part of carbon in the products that could not be analyzed by the micro GC, and the coke that was deposited on the reactor catalyst bed. Since the yields of C2H6, C3H8 and C4H10 were about zero, the results were just limited to the yields of CH4, CO and CO2. moles of gas produced × 100 moles of carbon fed
In the case of the bio-oil used here (CH1.87O0.754), the stoichiometry for the overall steam reforming reaction is defined as follows:
Other gases yieldð%Þ =
CH1:87 O0:754 þ 1:246H2 O↔CO2 þ 2:181H2
The S/C ratio was also defined as the number of moles of water fed to the moles of carbon in the feed (Eq. (10)). Note that the water present in the bio-oil, which was 23.1 wt%, was considered in the formula.
ð4Þ
This means that the overall stoichiometry gives a maximum hydrogen yield of 16.67 wt.% for the bio-oil used here. However, in actual conditions, the yield of hydrogen is lower than the stoichiometric maximum, because of the side reactions [27–29], such as the thermal decomposition (Eq. (5)), methanation reaction (Eq. (6)), and boudouard reaction (Eq. (7)). Cn Hm Ok →Cx Hy Oz + gasðH2 ; CO; CO2 ; CH4 ; :::Þ + coke
ð5Þ
CO + 3H2 ↔CH4 + H2 O
ð6Þ
2CO↔CO2 þ C
ð7Þ
N2 was used both as a carrier gas and an internal standard with knowledge of the N2 flow rate. The number of moles of produced gases per unit of time was calculated from the molar proportions with respect to the internal standard gas.
S
C =
moles of total water fed moles of carbon fed
ð9Þ
ð10Þ
For the reaction system, the bio-oil weight hourly space velocity (WbHSV) is defined by the formula given in Eq. (11) [21,29]: mass flowrate of bio−oil −1 = Wb HSV hr mass of catalyst
ð11Þ
3. Results and discussion 3.1. Characteristics of the Ni/ Al2O3 catalysts The catalysts prepared with different nickel contents were tested by TPR analysis. The resulting TPR profiles belonging to the Ni/Al2O3
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catalysts with different nickel contents are shown in Fig. 2. All the catalysts showed similar profiles which were mainly characterized by a sharp maximum at temperatures from 825 to 850 °C and one or two peaks at lower temperatures (300–450 °C). This means that, in the catalysts, different species of Ni with different reducibility were present [30]. Huang et al. performed the TPR test for a Ni/Al2O3 sample. The method employed for preparation of their catalyst [31] was similar to ours. They observed two peaks, one at a higher temperature (782 °C) and the second one at a lower temperature (398 °C). They postulated that the peak near to 350 °C could be ascribed to the reduction of nickel oxides and those beyond 700 °C could be ascribed to nickel aluminum spinel, NiAl2O4. Based on these findings, most of the nickels in our samples are in the form of NiAl2O4 (Fig. 2). Jeong et al. also observed the reduction of nickel oxide (NiO) species by H2 at two temperature regions: 400 and 875 °C. The first was assigned to the reduction of NiO clusters on Al2O3 and the latter to the NiAl2O4 [32]. This may be due to low pH, as mentioned in Section 2.2, the pH of the slurry was almost 2 when the catalysts were prepared. Chen et al. believed that impregnation of nickel salts on alumina supports leads to dissolution of alumina during impregnation at low pH and then incorporation into crystallized NiO precursors [33]. Table 2 summarizes the BET surface area, pore volume, and average pore size with different Ni contents. It is shown that the BET surface area and the total pore volume decreased with the impregnation of nickel nitrate on the support. These two properties of the catalysts gradually decreased further by increasing the Ni content, which suggested a negative effect of Ni on texture properties. However, the average pore sizes of the catalysts were relatively uniform for the first three catalysts, but decreased for the fourth catalyst. It seems that the impregnated Ni species may be mainly located on the inner surface of the pores of the alumina support; and, for the catalyst with the highest Ni content, some aggregations may occur. 3.2. Steam reforming of bio-oil over Ni/Al2O3 catalysts In the first part of experiment, Ni-based alumina catalysts with different Ni contents, as well as the alumina support itself, were tested; and, the yields of H2, CO2, CO and CH4 were calculated. Based on the results obtained, the most active catalyst for the production of hydrogen was selected for further investigation. The influence of the operational conditions including the temperature, S/C ratio and WbHSV on hydrogen production, was studied using the same catalyst. During the experiments, fluctuations of the produced component yields were observed over time. This may have been due to the feeding of the bio-oil and water by syringe pumps, which were not strictly continuous as bio-oil and water droplets were injected into the reactor every 5–10 s on average. Therefore, the reactor effluent gas concentrations showed some periodic variations with time. This phenomenon has also 90 80
Intensity
70 60 50
Table 2 Characteristics of Ni/Al2O3 with various Ni content. Catalyst Ni content (%)
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore size (Å)
Al2O3 Ni-Al-1 Ni-Al-2 Ni-Al-3 Ni-Al-4
255.00 164.00 152.45 133.48 107.36
1.14 0.74 0.68 0.62 0.34
135.61 135.73 143.82 96.91
0 5.6 10.7 14.1 18
been reported by other researchers [28]. During the experiments, the catalyst bed temperature showed variations of ±30 °C. This was due to the nature of the steam reforming reaction, which is strictly endothermic. The temperature decreased when the reaction occurred on the catalyst. 3.2.1. Influence of the Ni content A series of experiments were carried out by changing the nickel content of the catalysts without altering the calcinations and reduction conditions. Fig. 3 shows the results of the catalytic activity of the catalysts for the production of hydrogen versus time during isothermal reaction at 850 °C. It was seen that, except for Ni-Al-1, which experienced a sharp drop in the hydrogen yield from 80% to 40% in the first 30 min, the rest of the catalysts showed periodic fluctuations. In addition, for Al2O3 without any Ni content (Fig. 4), no significant changes were observed in the yields of hydrogen and even the other produced gases in the 1-h experiment. Table 3 shows the average yields of the gases in the exit stream of the reactor for a period of 1 h. The average hydrogen yield ranged from 46 to 65% for the four catalysts (Ni-Al-1 to Ni-Al-4), while a 32.5% H2 yield was achieved over the support at the same operational conditions, which was 14% less than that for the lowest active catalyst. Table 3 shows that the activity of the catalyst was not always enhanced with increased nickel content, and Ni-Al-3 was the most effective nickel content among the catalysts. The carbon conversion was relatively high over the catalysts, even the alumina itself. However, major changes in the selectivity were observed when alumina itself or each of the catalysts was employed. It was shown that, by increasing the Ni content, the yields of C1 and CO diminished, whereas the yield of CO2 increased from alumina with zero Ni content to Ni-Al-3 and then decreased. Looking at the CO/CO2 ratios, it was seen that this ratio decreased by increasing the Ni content, even for the Ni-Al-4 sample. It seems that the shift reaction rate increased when the Ni content was increased from zero to the third catalyst (Ni%=14.1%), but it did not change much from Ni-Al-3 to Ni-Al-4. It may be concluded that the best catalytic activity was found over Ni-Al-3, both in terms of hydrogen production and selectivity. In order to study the stability of the catalysts in the steam reforming of bio-oil, the activity of Ni-Al-3 catalyst was also tested for 3 h, at operational conditions of a bio-oil feed rate = 2.13 ml/h, water feed rate = 8.4 ml/h, S/C = 5, WbHSV = 13 1/h and temperature = 850 °C. The results are shown in Fig. 5. It was observed that the average yield of H2 slowly decreased by almost 12% after the 3 h. At the same time, a gradual increase of the CO yield along with a decrease of the CO2 yield indicated that the WGS reaction may not have proceeded well. In parallel, the methane yield was also increased. These observations reveal that the activity of the catalyst was reduced, due to coke deposition and catalyst deactivation.
40 30 20 10 0
0
100
200
300
400
500
600
700
800
900 1000
Temperature (OC) Fig. 2. Temperature-programmed reduction profiles of supported Ni catalysts.
3.2.2. Influence of temperature The Ni-Al-3 catalyst was tested at a series of temperatures, from 650 to almost 950 °C. Each experiment was carried out for 1 h, and the average yields are presented in Fig. 6. As shown, the hydrogen yield and carbon conversion increased with increasing reaction temperature. The CO yield was also enhanced by increasing temperature. These results indicated that WGS reaction equilibrium was not achieved at higher temperatures, due to the exothermic nature of this reaction. The same
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Fig. 3. Steam reforming of bio-oil over Ni/Al2O3 with different Ni loading: (a) Ni-Al-1, (b) Ni-Al-2, (c) Ni-Al-3, (d) Ni-Al-4. Operational conditions: nitrogen flow rate= 200 STP ml/min; bio-oil feed rate = 2.13 ml/h; water feed rate= 8.4 ml/h; S/C = 5; WbHSV = 13 1/h; temperature = 850 °C.
trend was also observed for CO2, except for a drop from 35 to 30% when the temperature increased from 850 to 955 °C. This may be due to the inverse WGS reaction, which results in CO2 and H2 consumption and, consequently, CO production. Based on these results, it is postulated that the hydrogen yield can be increased by adding a low-temperature WGS reactor downstream of the steam reformer reactor [28]. It has been reported that the addition of a WGS reactor after the reforming reactor, to convert CO via Eq. (2), can push the hydrogen yield to almost 100%, with only minor amounts of hydrogen lost to CH4 [27].
3.2.3. Influence of the S/C ratio The S/C ratio was adjusted by adding water. The effect of the S/C ratio on the H2 yield, as well as CO, CO2, CH4 yields and coke deposition (%), is presented in Fig. 7. The H2 yield was enhanced by increasing the S/C ratio from 1 to 2, whereas it leveled off by further increasing the S/C ratio. Domine et al. [24] investigated the effect of the S/C ratio on hydrogen production over Pt and Rh noble metal-based catalysts. They reported that the yield of H2 increased by increasing the S/C ratio over the Pt catalyst; whereas, over Rh-based catalyst, the hydrogen yield did not change much for an S/C ratio greater than 2.25. From their results, it can be concluded that the effect of the S/C ratio depends on the kind of metal sites of the catalyst. In addition, it was observed that the CO/CO2 ratio dramatically decreased when the S/C ratio was increased from 1 to 8. At the same time, the coke deposition on the catalyst also decreased. Coke deposition (%) is defined as the amount of coke deposited on the catalyst bed over the amount of bio-oil fed. Coke deposition on catalysts is one of the main drawbacks in the catalytic steam reforming of bio-oil. Rennard et al. [34] believed that this is due to the dehydration and polymerization of aromatic and unsaturated Table 3 Average yields of produced gases in a period of 1 h.
Fig. 4. Steam reforming of bio-oil over Al2O3. Operational conditions: nitrogen flow rate=200 STP ml/min; bio-oil feed rate=2.13 ml/h; water feed rate=8.4 ml/h; S/C=5; WbHSV=13 1/h; temperature=850 °C.
Catalyst
H2 %
C1 %
CO %
CO2 %
CO/CO2
Al2O3 Ni-Al-1 Ni-Al-2 Ni-Al-3 Ni-Al-4
32.5 46.2 51.8 65.3 54.7
9.6 7.7 3.9 2.3 1.1
42.7 42.6 29.5 26.5 16.8
11.5 16.4 19.2 34.8 27.25
3.72 2.60 1.53 0.76 0.62
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Fig. 5. Long-term activity of Ni-Al-3 for steam reforming of bio-oil. Operational conditions: nitrogen flow rate = 200 STP ml/min; bio-oil feed rate = 2.13 ml/h; water feed rate = 8.4 ml/h; S/C = 5; WbHSV = 13 1/h; temperature = 850 °C.
Fig. 7. Steam reforming of bio-oil over Ni-Al-3 at various S/C ratios. Operational conditions: nitrogen flow rate = 200 STP ml/min; bio-oil feed rate = 2.13 ml/h; water feed rate = 1.2–13.8 ml/h; WbHSV = 13 1/h; temperature = 850 °C.
components in the oil, especially in the lignin-derived fraction. Iojoiu et al. [20] reported that thermal decomposition of bio-oil occurred at 700 °C due to the thermal instability of bio-oil components, resulting in soot-like carbon deposition. It may be concluded that, in steam reforming, bio-oil compounds undergo thermal cracking to smaller gas molecules and coke before reaching the catalyst bed. Coke deposition not only decreases the hydrogen yield, but also leads to hard catalyst deactivation and reactor blockage. 3.2.4. Influence of WbHSV Considering the results obtained in Section 3.2.1, the Ni-Al-3 catalyst was selected for investigation of the influence of WbHSV on the activity of the catalyst at 850 °C and an S/C ratio of 5. The results obtained are presented in Fig. 8. It is obvious that the residence time of the reactants on catalyst bed decreased when WbHSV increased; therefore, the catalytic steam reforming efficiency of bio-oil components decreased. As a result, more reactants remained unconverted, which may condense with extra water or leave the system with exit gases. Consequently, the hydrogen yield decreased when the WbHSV increased. At the same time, the coke deposition did not change up to the WbHSV= 13, and then it increased dramatically up to double. 4. Conclusion A relatively high H2 yield (up to 73%) over Ni-based alumina catalysts was achieved by the steam reforming of bio-oil. Major changes in the selectivity were observed when alumina itself or catalysts with different
Fig. 8. Steam reforming of bio-oil over Ni-Al-3 at various WbHSV. Operational conditions: nitrogen flow rate = 200 STP ml/min; bio-oil feed rate = 2.13 ml/h; water feed rate = 8.4 ml/h; S/C = 5; temperature = 850 °C.
Ni content were employed. Alumina with a 14.1% Ni content gave the highest yield of hydrogen among the tested catalysts at the same operational conditions. The H2 yield increased with increasing temperature and decreasing WbHSV, whereas the effect of the S/C ratio was less pronounced. In the S/C ratio range of 1 to 2, the hydrogen yield slightly increased; however, when the S/C ratio was increased further, the S/C ratio did not affect on the H2 production yield. Gradual deactivation of the catalysts was observed when the duration of the experiment increased. Acknowledgement The financial supports of the Agriculture Funding Consortium, the Alberta Innovates - Bio Solutions, the Natural Science and Engineering Research council of Canada, and the Institute for Sustainable Energy, Environmental and Economy are thankfully acknowledged. References
Fig. 6. Steam reforming of bio-oil over Ni-Al-3 at various temperatures. Operational conditions: nitrogen flow rate=200 STP ml/min; bio-oil feed rate =2.13 cc/h; water feed rate =8.4 cc/h; S/C= 5; WbHSV= 13 1/h.
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F. Seyedeyn-Azad et al. / Fuel Processing Technology 92 (2011) 563–569 [4] L.P.R. Profeti, E.A. Ticianelli, E.M. Assaf, Co/Al2O3 catalysts promoted with noble metals for production of hydrogen by methane steam reforming, Fuel 87 (2008) 2076–2081. [5] K.M. Hardiman, T.T. Ying, A.A. Adesina, E.M. Kennedy, B.Z. Dlugorski, Performance of a Co-Ni catalyst for propane reforming under low steam-to-carbon ratios, Chem. Eng. J. 102 (2004) 119–130. [6] J.G. Seo, M.H. Youn, J.C. Jung, I.K. Song, Effect of preparation method of mesoporous Ni–Al2O3 catalysts on their catalytic activity for hydrogen production by steam reforming of liquefied natural gas (LNG), Int. J. Hydrogen Energy 34 (2009) 5409–5416. [7] K. Kazuhiro, K. Aika, K. Seshan, L. Lefferts, Catalyst deactivation during steam reforming of acetic acid over Pt/ZrO2, Chem. Eng. J. 120 (2006) 133–137. [8] E.C. Vagia, A.A. Lemonidou, Hydrogen production via steam reforming of bio-oil components over calcium aluminate supported nickel and noble metal catalysts, Appl. Catal. A Gen. 351 (2008) 111–121. [9] M. Marquevich, S. Czernik, E. Chornet, D. Montane, Hydrogen from biomass: steam reforming of model compounds of fast-pyrolysis oil, Energ. Fuel 13 (1999) 1160–1166. [10] K. Takanabe, K. Aika, K. Seshan, L. Lefferts, Sustainable hydrogen from bio-oilSteam reforming of acetic acid as a model oxygenate, J. Catalysis 227 (2004) 101–108. [11] B.M. Güell, I. Babich, K.P. Nichols, J.G.E. Gardeniers, L. Lefferts, K. Seshan, Design of a stable steam reforming catalyst—a promising route to sustainable hydrogen from biomass oxygenates, Appl. Catal. B Environ. 90 (2009) 38–44. [12] C. Li, D. Hirabayashi, K. Suzuki, Development of new nickel based catalyst for biomass tar steam reforming producing, H2-rich syngas, Fuel Process. Technol. 90 (2009) 790–796. [13] S. Czernik, R. Evans, R. French, Hydrogen from biomass-production by steam reforming of biomass pyrolysis oil, Catal. Today 129 (2007) 265–268. [14] J.A. Medrano, M. Oliva, J. Ruiz, L. García, J. Arauzo, Hydrogen from aqueous fraction of biomass pyrolysis liquids by catalytic steam reforming in fluidized bed, Energy (in press), available online. [15] S. Czernik, R. French, C. Feik, E. Chornet, Hydrogen by catalytic steam reforming of liquid byproducts from biomass thermoconversion processes, Ind. Eng. Chem. Res. 41 (2002) 4209–4215. [16] A.C. Basagiannis, X.E. Verykios, Steam reforming of the aqueous fraction of bio-oil over structured Ru/MgO/Al2O3 catalysts, Catal. Today 127 (2007) 256–264. [17] L. Garcia, R. French, S. Czernik, E. Chornet, Catalytic steam reforming of bio-oils for the production of hydrogen: effects of catalyst composition, Appl. Catal. A Gen. 201 (2000) 225–239. [18] N. Palmeri, V. Chiodo, S. Freni, F. Frusteri, J.C.J. Bart, S. Cavallaro, Hydrogen from oxygenated solvents by steam reforming on Ni/Al2O3 catalyst, Int. J. Hydrogen Energy 33 (2008) 6627–6634. [19] P.N. Kechagiopoulos, S.S. Voutetakis, A.A. Lemonidou, I.A. Vasalos, Hydrogen production via reforming of the aqueous phase of bio-oil over Ni/olivine catalysts in a spouted bed reactor, Ind. Eng. Chem. Res. 48 (2009) 1400–1408.
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Biomass to hydrogen via catalytic steam reforming of bio-oil over Ni-supported alumina catalysts F. Seyedeyn-Azad, E. Salehi, J. Abedi ⁎, T. Harding Chemical and Petroleum Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada
a r t i c l e
i n f o
Article history: Received 25 June 2010 Accepted 17 November 2010 Available online 14 December 2010 Keywords: Hydrogen production Pyrolysis bio-oil Catalytic steam reforming Ni/Al2O3
a b s t r a c t Production of hydrogen (H2) from catalytic steam reforming of bio-oil was investigated in a fixed bed tubular flow reactor over nickel/alumina (Ni/Al2O3) supported catalysts at different conditions. The features of the steam reforming of bio-oil, including the effects of metal content, reaction temperature, WbHSV (defined as the mass flow rate of bio-oil per mass of catalyst) and S/C ratio (the molar ratio of steam to carbon fed) on the hydrogen yield were investigated. Carbon conversion (moles of carbon in the outlet gases to moles of the carbon feed) was also studied, and the outlet gas distributions were obtained. It was revealed that the Al2O3 with 14.1% Ni content gave the highest yield of hydrogen (73%) among the catalysts tested, and the best carbon conversion was 79% under the steam reforming conditions of S/C = 5, WbHSV = 13 1/h and temperature = 950 °C. The H2 yield increased with increasing temperature and decreasing WbHSV; whereas the effect of the S/C ratio was less pronounced. In the S/C ratio range of 1 to 2, the hydrogen yield was slightly increased, but when the S/C ratio was increased further, it did not have an effect on the H2 production yield. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The demand for fossil fuel, considering the available reserves and the pollution caused by continuous consumption, makes hydrogen an attractive substitute energy source [1]. With respect to global issues, hydrogen production from bio-oil produced from fast pyrolysis of biomass is a viable process [2]. Catalytic steam reforming of methane [3,4], propane [5], liquefied natural gas (LNG) [6], ethanol [1], model compounds, such as acetic acid and acetone [7–11], toluene [12], diluted bio-oil with methanol [13] over different catalysts has been widely investigated. A great deal of research has also been carried out on the production of hydrogen by steam reforming of the aqueous fraction of bio-oil [14–19]. By adding water, bio-oil breaks into two immiscible phases: the first phase (aqueous) forms with carbohydrate-derived compounds and light oxygenated compounds; whereas, the second phase (hydrophobic) contains aromatic and lignin derivatives [20]. It is worth noting that the energy yield decreases when the aqueous phase of bio-oil is used, due to the requirement of the primary separation process. Direct use of crude bio-oil has also been studied in sequential cracking processes. Injoiu et al. employed different catalysts, including noble metal catalysts supported on ceria-zirconia, in such a process [20]. Although the high cost of noble metals makes commercial applications questionable, slow deactivation phenomena with time were also observed.
⁎ Corresponding author. Tel.: +1 403 220 5594. E-mail address: [email protected] (J. Abedi). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.11.012
Steam reforming of bio-oil itself was also investigated over the nickel-based catalyst nickel/magnesia (Ni/MgO) in a two-stage fixed bed reactor system [21]. In the first stage, dolomite was used for the primary steam reforming of bio-oil; and, in the second stage, the nickel-based catalyst Ni/MgO was employed for the conversion of methane, in order to increase the purity and yield of the desired product. It was reported that, for efficient conversion of bio-oil to a favored gas product, temperatures greater than 850 °C and S/C ratio greater than 12 were required. The temperatures of the first and the second stages were set at 900 and 850 °C, respectively. The system also had to work at a relatively low GHSV (defined as gas hourly space velocity) to be able to convert methane. Davidian et al. [22] also employed a sequential cracking/reforming process. The H2 concentration in the exit gas stream was 45–50 vol.% over Ni/Al2O3 and nickel-potassium/lithium oxide-alumina (Ni-K/Li2O3-Al2O3) catalysts. Kan et al. [23] obtained a higher H2 yield (87.6%) and tried to reduce the severe deactivation of the catalyst in the production of hydrogen from crude bio-oil. They proposed an efficient approach by using crude bio-oil via the integrative process between gasification and current-enhanced catalytic steam reforming. They used a nickel/copper/zinc/aluminum (NiCuZnAl) reforming catalyst in the downstream process. Domine et al. [24] also investigated steam reforming of bio-oil over monolithic platinum (Pt) and rhodium (Rh)-based catalysts using the steam reforming and sequential cracking processes. They reported that the highest yield of H2 was 70% with the Pt catalyst at an S/C ratio of 10 at 780 °C. According to our knowledge, few studies have been performed on the steam reforming of bio-oil itself in a one-stage process, particularly over
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Ni-supported alumina catalysts. This is partly due to the experimental difficulties incorporated in the testing of a thick bio-oil phase. It is well known that nickel-based catalysts give a high yield of hydrogen for the steam reforming of the model compounds, as well as the aqueous phase of bio-oil [25,26]. Thus, more exploitation of the non-noble metal Ni/Al2O3 catalyst is necessary and significant, where correlative studies on steam reforming of bio-oil itself are very limited. In this paper, we report the investigations performed for hydrogen production over Ni/Al2O3 supported catalysts by steam reforming of the whole bio-oil as a feedstock (not only the aqueous fraction) which is commonly used. The effect of various influential parameters on the hydrogen production is also reported herein. 2. Experimental 2.1. Feedstock and characterization The bio-oil, a dark brown liquid fluid with a strong smoky odor, used in this research was provided by Biomass Technology Group (BTG), The Netherlands. The main elemental composition of the bio-oil was carbon (C), hydrogen (H), nitrogen (N) and oxygen (O) (Table 1); therefore, the oxygenated organic compounds in the bio-oil were generally described by the average chemical formula of CH1.87O0.754, including water. The elemental analysis was performed using a PerkinElmer model 2400 CHN analyzer. The bio-oil also contained impurities, such as sulfur, and traces of metals, such as potassium, magnesium and calcium, which have not been analyzed here. Table 1 also shows the water content, pH, density and higher heating value (HHV) of the bio-oil. The water content of the bio-oil was measured by applying the Karl-Fisher titration method using a Mettler Toledo DL32 Coulometric Titrator. The density of bio-oil was measured with a density measurement bottle, and a calorific value for the bio-oil was also determined using a bomb calorimeter (Parr model 1266). 2.2. Catalyst preparation and characterization γ-Al2O3 and Ni(NO3)2. 6H2O were purchased from Alfa Aesar Company. The γ-Al2O3 used as a support was pretreated at 800 °C for 6 h. It was crushed to a size of 180–250 μm. A known amount of Ni (NO3)2.6H2O was dissolved in water and the pretreated γ-Al2O3 was added to the solution. The pH of the nickel electrolytes and the bio-oil was measured using an Oakton Instruments pH meter calibrated by buffer solutions with pH 2 and 10. The pH of the obtained slurry was about 2. The slurry was continuously stirred at room temperature for 3 h. It was then dried for 48 h in an oven at 120 °C. To make nickel nitrate decompose to nickel oxide, the dried sample was calcined at 700 °C for 3.5 h. Textural properties of the catalysts were obtained by using a Micromeritics Tristar II 3020 instrument. About 100 mg of each of the samples was pretreated overnight at 150 °C. Nitrogen physisorption
experiments were carried out at 77 K to obtain BET surface area, pore volumes and pore size distribution. Temperature-programmed reduction (TPR) of the catalysts was carried out using a Quantachrome CHEMBET 3000 instrument. About 50–80 mg of each sample was placed in a U-shaped reactor. Quartz wool was used to keep the solid sample in place. First, the sample was pretreated under nitrogen (N2) flow (15 ml/min) at 150 °C overnight. The catalyst was then oxidized using air flow (25 ml/min) and heating to 400 °C at a rate of 10 °C/min. The temperature was held at 400 °C for 1 h. Argon (Ar) was then used to flush out any traces of air, while the system was cooled to ambient temperature. TPR analysis was then carried out using 10% H2 in Ar (flow rate 15 ml/min). The catalyst was heated up to 900 °C at a heating rate of 10 °C/min. The signals were recorded using a thermal conductivity detector (TCD). The system was equipped with a cold trap to prevent moisture from entering the detector.
2.3. Apparatus An experimental setup (Fig. 1) was designed and developed to operate at atmospheric pressure and temperatures up to 1000 °C. Two mass flow controllers (MKS, Model M100B) were employed to control and measure N2 (carrier gas) and H2 (reduction gas). A tubular furnace (Thermolyne, Model F21135) was used to provide heat for the system. A tubular reactor with an internal diameter of 1 cm and a height of 43 cm was constructed from stainless steel. A cross shape was welded inside the reactor as a stand for the catalyst bed in the middle of the reactor. Two K-type thermocouples were employed to monitor the temperature of the catalyst bed and the condenser. The operational parameters (temperature, and gas flow rate) were controlled and monitored with LabView software (version 8.5), and recorded data were saved from each run in a data file. The excess steam was recovered by a glass condenser. The coolant was provided by a water-glycol bath circulator (VWR, Model 1150 S). Two high-pressure syringe pumps (ISCO, Model 500D) were used to feed bio-oil and water to the fixed bed catalytic reactor. Water and bio-oil were injected into the reactor through two capillaries that were surrounded by a cooling jacket. This was done to prevent the polymerization of the bio-oil before entering the reactor because bio-oil polymerizes at 80 °C and decomposes upon heating [24]. The needle of the water syringe was located exactly above the needle of the bio-oil syringe. Thus, water droplets entrained the bio-oil droplets on the way to the catalyst surface. After passing through the condenser, effluent gas from the reactor was sent to an on-line micro gas chromatograph (GC) (Varian, Model CP-4900) that was equipped with a TCD. The GC was calibrated by a calibration gas mixture of N2, H2, carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10).
2.4. Procedure Table 1 Characteristics of the BTG bio-oil. Elemental analysis wt (%) Carbon Hydrogen Nitrogen Oxygena Water content wt (%) pH Density (kg/m3) HHV (MJ/kg) a
By difference.
45.83 7.15 0.87 46.15 23.1 2.1 1225.00 17.5
In a typical experiment, 0.2 g of catalyst was loaded into the reactor. For each experiment run, a thin layer of glass wool was placed on the cross-shaped stand to hold the catalyst particles. The electrical furnace was heated to the required temperature. The thermocouple measured the bed temperature every second, and the micro GC took samples of the produced gas, downstream of the condenser, every 4 min. Based on the results obtained from the TPR tests, the catalysts were reduced in situ at 850 °C for 2 h with 400 STP ml/min of a 50 vol.%H2/N2 mixture followed by flashing in nitrogen at 850 °C prior to each run of the experiment. The effect of the Ni content, temperature, WbHSV and the S/C ratio on the yield of H2, CO2, CO and CH4 was investigated in a systematic manner.
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Fig. 1. Schematic diagram of the catalytic steam reforming system: (1), (2) mass flow controllers; (3) bio-oil syringe pump; (4) water syringe pump; (5) reactor equipped with a TC.
2.5. Data analysis Steam reforming of bio-oil can be simplified as the catalytic steam reforming of oxygenated organic compounds (CnHmOk) by the following reaction:
ð
Cn Hm Ok þ ðn kÞH2 O↔nCO þ n þ
Þ
m k H2 2
ð1Þ
The produced gas mixture (CO and H2) is commonly named synthesis gas. CO and the excess steam used in the process further undergo a reaction, named the water-gas shift (WGS) reaction: nCO + nH2 O↔nCO2 + nH2
ð2Þ
Therefore, the overall process can be represented as follows: m Cn Hm Ok + ð2n−kÞH2 O↔nCO2 + 2n + −k H2 2
Different Ni catalysts were compared regarding their activities in terms of H2, CH4, CO and CO2 yields. The hydrogen yield (Eq. (8)) was defined as the ratio of the number of moles of produced hydrogen to the theoretical quantity of hydrogen obtained when complete reforming to CO2 and H2 takes place. Eq. (8) shows that produced hydrogen originates not only from the bio-oil but also from the steam.
ð3Þ
H2 yieldð%Þ =
moles of hydrogen produced × 100 2:181 × moles of carbon fed
ð8Þ
The yield of the other gases (CH4, C2H6, C3H8, C4H10, CO and CO2) was defined (Eq. (9)) as the number of moles of gas obtained per mole of carbon feed. It was obvious that the sum of the CO, C2H6, C3H8, C4H10, CO2 and CH4 yields, called carbon conversion, could not be 100%. The difference related to that part of carbon in the products that could not be analyzed by the micro GC, and the coke that was deposited on the reactor catalyst bed. Since the yields of C2H6, C3H8 and C4H10 were about zero, the results were just limited to the yields of CH4, CO and CO2. moles of gas produced × 100 moles of carbon fed
In the case of the bio-oil used here (CH1.87O0.754), the stoichiometry for the overall steam reforming reaction is defined as follows:
Other gases yieldð%Þ =
CH1:87 O0:754 þ 1:246H2 O↔CO2 þ 2:181H2
The S/C ratio was also defined as the number of moles of water fed to the moles of carbon in the feed (Eq. (10)). Note that the water present in the bio-oil, which was 23.1 wt%, was considered in the formula.
ð4Þ
This means that the overall stoichiometry gives a maximum hydrogen yield of 16.67 wt.% for the bio-oil used here. However, in actual conditions, the yield of hydrogen is lower than the stoichiometric maximum, because of the side reactions [27–29], such as the thermal decomposition (Eq. (5)), methanation reaction (Eq. (6)), and boudouard reaction (Eq. (7)). Cn Hm Ok →Cx Hy Oz + gasðH2 ; CO; CO2 ; CH4 ; :::Þ + coke
ð5Þ
CO + 3H2 ↔CH4 + H2 O
ð6Þ
2CO↔CO2 þ C
ð7Þ
N2 was used both as a carrier gas and an internal standard with knowledge of the N2 flow rate. The number of moles of produced gases per unit of time was calculated from the molar proportions with respect to the internal standard gas.
S
C =
moles of total water fed moles of carbon fed
ð9Þ
ð10Þ
For the reaction system, the bio-oil weight hourly space velocity (WbHSV) is defined by the formula given in Eq. (11) [21,29]: mass flowrate of bio−oil −1 = Wb HSV hr mass of catalyst
ð11Þ
3. Results and discussion 3.1. Characteristics of the Ni/ Al2O3 catalysts The catalysts prepared with different nickel contents were tested by TPR analysis. The resulting TPR profiles belonging to the Ni/Al2O3
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catalysts with different nickel contents are shown in Fig. 2. All the catalysts showed similar profiles which were mainly characterized by a sharp maximum at temperatures from 825 to 850 °C and one or two peaks at lower temperatures (300–450 °C). This means that, in the catalysts, different species of Ni with different reducibility were present [30]. Huang et al. performed the TPR test for a Ni/Al2O3 sample. The method employed for preparation of their catalyst [31] was similar to ours. They observed two peaks, one at a higher temperature (782 °C) and the second one at a lower temperature (398 °C). They postulated that the peak near to 350 °C could be ascribed to the reduction of nickel oxides and those beyond 700 °C could be ascribed to nickel aluminum spinel, NiAl2O4. Based on these findings, most of the nickels in our samples are in the form of NiAl2O4 (Fig. 2). Jeong et al. also observed the reduction of nickel oxide (NiO) species by H2 at two temperature regions: 400 and 875 °C. The first was assigned to the reduction of NiO clusters on Al2O3 and the latter to the NiAl2O4 [32]. This may be due to low pH, as mentioned in Section 2.2, the pH of the slurry was almost 2 when the catalysts were prepared. Chen et al. believed that impregnation of nickel salts on alumina supports leads to dissolution of alumina during impregnation at low pH and then incorporation into crystallized NiO precursors [33]. Table 2 summarizes the BET surface area, pore volume, and average pore size with different Ni contents. It is shown that the BET surface area and the total pore volume decreased with the impregnation of nickel nitrate on the support. These two properties of the catalysts gradually decreased further by increasing the Ni content, which suggested a negative effect of Ni on texture properties. However, the average pore sizes of the catalysts were relatively uniform for the first three catalysts, but decreased for the fourth catalyst. It seems that the impregnated Ni species may be mainly located on the inner surface of the pores of the alumina support; and, for the catalyst with the highest Ni content, some aggregations may occur. 3.2. Steam reforming of bio-oil over Ni/Al2O3 catalysts In the first part of experiment, Ni-based alumina catalysts with different Ni contents, as well as the alumina support itself, were tested; and, the yields of H2, CO2, CO and CH4 were calculated. Based on the results obtained, the most active catalyst for the production of hydrogen was selected for further investigation. The influence of the operational conditions including the temperature, S/C ratio and WbHSV on hydrogen production, was studied using the same catalyst. During the experiments, fluctuations of the produced component yields were observed over time. This may have been due to the feeding of the bio-oil and water by syringe pumps, which were not strictly continuous as bio-oil and water droplets were injected into the reactor every 5–10 s on average. Therefore, the reactor effluent gas concentrations showed some periodic variations with time. This phenomenon has also 90 80
Intensity
70 60 50
Table 2 Characteristics of Ni/Al2O3 with various Ni content. Catalyst Ni content (%)
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore size (Å)
Al2O3 Ni-Al-1 Ni-Al-2 Ni-Al-3 Ni-Al-4
255.00 164.00 152.45 133.48 107.36
1.14 0.74 0.68 0.62 0.34
135.61 135.73 143.82 96.91
0 5.6 10.7 14.1 18
been reported by other researchers [28]. During the experiments, the catalyst bed temperature showed variations of ±30 °C. This was due to the nature of the steam reforming reaction, which is strictly endothermic. The temperature decreased when the reaction occurred on the catalyst. 3.2.1. Influence of the Ni content A series of experiments were carried out by changing the nickel content of the catalysts without altering the calcinations and reduction conditions. Fig. 3 shows the results of the catalytic activity of the catalysts for the production of hydrogen versus time during isothermal reaction at 850 °C. It was seen that, except for Ni-Al-1, which experienced a sharp drop in the hydrogen yield from 80% to 40% in the first 30 min, the rest of the catalysts showed periodic fluctuations. In addition, for Al2O3 without any Ni content (Fig. 4), no significant changes were observed in the yields of hydrogen and even the other produced gases in the 1-h experiment. Table 3 shows the average yields of the gases in the exit stream of the reactor for a period of 1 h. The average hydrogen yield ranged from 46 to 65% for the four catalysts (Ni-Al-1 to Ni-Al-4), while a 32.5% H2 yield was achieved over the support at the same operational conditions, which was 14% less than that for the lowest active catalyst. Table 3 shows that the activity of the catalyst was not always enhanced with increased nickel content, and Ni-Al-3 was the most effective nickel content among the catalysts. The carbon conversion was relatively high over the catalysts, even the alumina itself. However, major changes in the selectivity were observed when alumina itself or each of the catalysts was employed. It was shown that, by increasing the Ni content, the yields of C1 and CO diminished, whereas the yield of CO2 increased from alumina with zero Ni content to Ni-Al-3 and then decreased. Looking at the CO/CO2 ratios, it was seen that this ratio decreased by increasing the Ni content, even for the Ni-Al-4 sample. It seems that the shift reaction rate increased when the Ni content was increased from zero to the third catalyst (Ni%=14.1%), but it did not change much from Ni-Al-3 to Ni-Al-4. It may be concluded that the best catalytic activity was found over Ni-Al-3, both in terms of hydrogen production and selectivity. In order to study the stability of the catalysts in the steam reforming of bio-oil, the activity of Ni-Al-3 catalyst was also tested for 3 h, at operational conditions of a bio-oil feed rate = 2.13 ml/h, water feed rate = 8.4 ml/h, S/C = 5, WbHSV = 13 1/h and temperature = 850 °C. The results are shown in Fig. 5. It was observed that the average yield of H2 slowly decreased by almost 12% after the 3 h. At the same time, a gradual increase of the CO yield along with a decrease of the CO2 yield indicated that the WGS reaction may not have proceeded well. In parallel, the methane yield was also increased. These observations reveal that the activity of the catalyst was reduced, due to coke deposition and catalyst deactivation.
40 30 20 10 0
0
100
200
300
400
500
600
700
800
900 1000
Temperature (OC) Fig. 2. Temperature-programmed reduction profiles of supported Ni catalysts.
3.2.2. Influence of temperature The Ni-Al-3 catalyst was tested at a series of temperatures, from 650 to almost 950 °C. Each experiment was carried out for 1 h, and the average yields are presented in Fig. 6. As shown, the hydrogen yield and carbon conversion increased with increasing reaction temperature. The CO yield was also enhanced by increasing temperature. These results indicated that WGS reaction equilibrium was not achieved at higher temperatures, due to the exothermic nature of this reaction. The same
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Fig. 3. Steam reforming of bio-oil over Ni/Al2O3 with different Ni loading: (a) Ni-Al-1, (b) Ni-Al-2, (c) Ni-Al-3, (d) Ni-Al-4. Operational conditions: nitrogen flow rate= 200 STP ml/min; bio-oil feed rate = 2.13 ml/h; water feed rate= 8.4 ml/h; S/C = 5; WbHSV = 13 1/h; temperature = 850 °C.
trend was also observed for CO2, except for a drop from 35 to 30% when the temperature increased from 850 to 955 °C. This may be due to the inverse WGS reaction, which results in CO2 and H2 consumption and, consequently, CO production. Based on these results, it is postulated that the hydrogen yield can be increased by adding a low-temperature WGS reactor downstream of the steam reformer reactor [28]. It has been reported that the addition of a WGS reactor after the reforming reactor, to convert CO via Eq. (2), can push the hydrogen yield to almost 100%, with only minor amounts of hydrogen lost to CH4 [27].
3.2.3. Influence of the S/C ratio The S/C ratio was adjusted by adding water. The effect of the S/C ratio on the H2 yield, as well as CO, CO2, CH4 yields and coke deposition (%), is presented in Fig. 7. The H2 yield was enhanced by increasing the S/C ratio from 1 to 2, whereas it leveled off by further increasing the S/C ratio. Domine et al. [24] investigated the effect of the S/C ratio on hydrogen production over Pt and Rh noble metal-based catalysts. They reported that the yield of H2 increased by increasing the S/C ratio over the Pt catalyst; whereas, over Rh-based catalyst, the hydrogen yield did not change much for an S/C ratio greater than 2.25. From their results, it can be concluded that the effect of the S/C ratio depends on the kind of metal sites of the catalyst. In addition, it was observed that the CO/CO2 ratio dramatically decreased when the S/C ratio was increased from 1 to 8. At the same time, the coke deposition on the catalyst also decreased. Coke deposition (%) is defined as the amount of coke deposited on the catalyst bed over the amount of bio-oil fed. Coke deposition on catalysts is one of the main drawbacks in the catalytic steam reforming of bio-oil. Rennard et al. [34] believed that this is due to the dehydration and polymerization of aromatic and unsaturated Table 3 Average yields of produced gases in a period of 1 h.
Fig. 4. Steam reforming of bio-oil over Al2O3. Operational conditions: nitrogen flow rate=200 STP ml/min; bio-oil feed rate=2.13 ml/h; water feed rate=8.4 ml/h; S/C=5; WbHSV=13 1/h; temperature=850 °C.
Catalyst
H2 %
C1 %
CO %
CO2 %
CO/CO2
Al2O3 Ni-Al-1 Ni-Al-2 Ni-Al-3 Ni-Al-4
32.5 46.2 51.8 65.3 54.7
9.6 7.7 3.9 2.3 1.1
42.7 42.6 29.5 26.5 16.8
11.5 16.4 19.2 34.8 27.25
3.72 2.60 1.53 0.76 0.62
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Fig. 5. Long-term activity of Ni-Al-3 for steam reforming of bio-oil. Operational conditions: nitrogen flow rate = 200 STP ml/min; bio-oil feed rate = 2.13 ml/h; water feed rate = 8.4 ml/h; S/C = 5; WbHSV = 13 1/h; temperature = 850 °C.
Fig. 7. Steam reforming of bio-oil over Ni-Al-3 at various S/C ratios. Operational conditions: nitrogen flow rate = 200 STP ml/min; bio-oil feed rate = 2.13 ml/h; water feed rate = 1.2–13.8 ml/h; WbHSV = 13 1/h; temperature = 850 °C.
components in the oil, especially in the lignin-derived fraction. Iojoiu et al. [20] reported that thermal decomposition of bio-oil occurred at 700 °C due to the thermal instability of bio-oil components, resulting in soot-like carbon deposition. It may be concluded that, in steam reforming, bio-oil compounds undergo thermal cracking to smaller gas molecules and coke before reaching the catalyst bed. Coke deposition not only decreases the hydrogen yield, but also leads to hard catalyst deactivation and reactor blockage. 3.2.4. Influence of WbHSV Considering the results obtained in Section 3.2.1, the Ni-Al-3 catalyst was selected for investigation of the influence of WbHSV on the activity of the catalyst at 850 °C and an S/C ratio of 5. The results obtained are presented in Fig. 8. It is obvious that the residence time of the reactants on catalyst bed decreased when WbHSV increased; therefore, the catalytic steam reforming efficiency of bio-oil components decreased. As a result, more reactants remained unconverted, which may condense with extra water or leave the system with exit gases. Consequently, the hydrogen yield decreased when the WbHSV increased. At the same time, the coke deposition did not change up to the WbHSV= 13, and then it increased dramatically up to double. 4. Conclusion A relatively high H2 yield (up to 73%) over Ni-based alumina catalysts was achieved by the steam reforming of bio-oil. Major changes in the selectivity were observed when alumina itself or catalysts with different
Fig. 8. Steam reforming of bio-oil over Ni-Al-3 at various WbHSV. Operational conditions: nitrogen flow rate = 200 STP ml/min; bio-oil feed rate = 2.13 ml/h; water feed rate = 8.4 ml/h; S/C = 5; temperature = 850 °C.
Ni content were employed. Alumina with a 14.1% Ni content gave the highest yield of hydrogen among the tested catalysts at the same operational conditions. The H2 yield increased with increasing temperature and decreasing WbHSV, whereas the effect of the S/C ratio was less pronounced. In the S/C ratio range of 1 to 2, the hydrogen yield slightly increased; however, when the S/C ratio was increased further, the S/C ratio did not affect on the H2 production yield. Gradual deactivation of the catalysts was observed when the duration of the experiment increased. Acknowledgement The financial supports of the Agriculture Funding Consortium, the Alberta Innovates - Bio Solutions, the Natural Science and Engineering Research council of Canada, and the Institute for Sustainable Energy, Environmental and Economy are thankfully acknowledged. References
Fig. 6. Steam reforming of bio-oil over Ni-Al-3 at various temperatures. Operational conditions: nitrogen flow rate=200 STP ml/min; bio-oil feed rate =2.13 cc/h; water feed rate =8.4 cc/h; S/C= 5; WbHSV= 13 1/h.
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