Hydrogen Production by Catalytic Steam Reforming of Acetol, a Model

Ind. Eng. Chem. Res. 2007, 46, 2399-2406

2399

Hydrogen Production by Catalytic Steam Reforming of Acetol, a Model Compound of Bio-Oil M. Carmen Ramos, Ana I. Navascue´ s, Lucıá Garcıá,* and Rafael Bilbao Aragon Institute of Engineering Research, Department of Chemical and EnVironmental Engineering, UniVersity of Zaragoza, Marıá de Luna 3, 50018 Zaragoza, Spain

Hydrogen can be produced by catalytic steam reforming of bio-oil or its fractions. Bio-oil is a complex mixture of a large number of compounds derived from fast pyrolysis of biomass. Acetol has been selected as a model compound. Steam reforming of acetol has been studied in a fluidized bed reactor using coprecipitated Ni-Al catalysts, some promoted with lanthanum and cobalt. Noncatalytic experiments have been performed from 450 to 650 °C. Catalytic experiments have been carried out at 600 and 650 °C in order to analyze the influence of the catalyst weight/acetol flow rate (W/mAc) ratio on gas yields. The influence of the steam to carbon molar (S/C) ratio and the catalyst composition on gas yields has also been studied. The presence of the catalyst increases H2, CO2, and total gas yields while CH4, CO, and C2 yields decrease. An increase in the S/C ratio at 650 °C increases H2, CO2, and total gas yields and carbon conversion to gas. The presence of lanthanum in Ni-Al coprecipitated catalysts increases CH4, CO2, C2, and total gas yields as well as carbon conversion to gas. Ni-Co-Al catalysts present the lowest values of carbon conversion to gas. Hydrogen yields obtained with the catalysts tested follow this sequence: Ni-Al ) Ni-Co-Al (Co/Ni ) 0.25) > Ni-Co-Al (Co/Ni ) 0.025) > Ni-Al-La (4 wt % La2O3) > Ni-Al-La (8 wt % La2O3) > Ni-Al-La (12 wt % La2O3). 1. Introduction Nowadays there is increasing interest in hydrogen production from renewable sources. One route is the steam reforming of various liquid and vapor waste streams such as trap grease,1 plastics,2 glycerin,3 and vegetable oils.4 Catalytic steam reforming of pyrolysis oils has been also explored.5-7 This process begins with a fast pyrolysis of biomass to produce bio-oil, which (either as a whole or using specific fractions) can be converted to hydrogen via catalytic steam reforming followed, if necessary, by a shift conversion step. Bio-oil is a complex mixture of a large number of compounds, including aldehydes, alcohols, ketones, and acids, as well as more-complex carbohydrate- and lignin-derived oligomeric materials emulsified with water. Two major characteristics of this bio-oil are high oxygen content and high density (much higher than wood). Another property is a limited water solubility.8 By simply adding water, the bio-oil separates into a waterrich phase, that contains mostly carbohydrate-derived compounds, and a hydrophobic phase that is composed mainly of lignin-derived oligomers. Given economic considerations, the following application of bio-oil for hydrogen production is proposed: the aqueous fraction is steam reformed to produce hydrogen, and the hydrophobic phase can then be used as a replacement for phenol in phenol formaldehyde resins9 or converted into aromatic hydrocarbons and ethers for use as high-octane gasolineblending components.10 Because of the complexity of bio-oil, a large number of papers in the literature use model compounds to study the steam reforming of bio-oil or its fractions. Acetic acid has been widely studied in the literature5,11-18 as a representative of organic acids and as a majority compound. Other model compounds used have been hydroxyacetaldehyde,11,5 m-cresol, benzyl ether and sug* To whom correspondence should be addressed. Tel.: 34-976762194. Fax: 34- 976-761879. E-mail: [email protected].

ars,12 acetone, ethanol, and phenol.15 The work of Wang et al.5 uses many model compounds of bio-oil, among them acetone, glycerol, anisole, and guaiacol. This paper is a part of a more general study about hydrogen production by steam reforming of the bio-oil aqueous fraction. The first paper studied steam reforming of acetic acid as representative of organic acids.14 We selected one of the compounds present in the bio-oil aqueous fraction, acetol, also called hydroxyacetone. This is a three-carbon compound with alcohol and ketone groups. This model compound is present at around 0.014 g/g biomass, m.f., and it can be considered between the fifth and eighth of majority compounds.8 In the IEA-EU round robin,19 pyrolysis liquids were analyzed in the year 2000. Acetol was included in the analyses of three of four laboratories, and in the pyrolysis liquids produced by four different chemically characterized processes. At various laboratories, acetol was one of the majority compounds of the group of aldehydes, ketones, and alcohols, exceeded only by hydroxyacetaldehyde, formaldehyde, and butanol in some analyses. The amount of acetol was up to 7.82 wt % based on wet liquid. Coprecipitated Ni-Al catalysts have been selected because of their mechanical strength and suitable performance in biomass pyrolysis20,21 and gasification.22,23 Cobalt and lanthanum have been chosen as promoters due to their beneficial effects observed in steam reforming of the aqueous fraction of bio-oil.7 Coprecipitated Ni-Al-La catalysts have also been studied in the catalytic steam gasification of pine sawdust.24 Recently, acetic acid has been steam reformed using Fe-Co catalysts.16 In the present paper, steam reforming of acetol has been studied in a bench scale installation using a fluidized bed reactor. Noncatalytic experiments of acetol steam reforming from 450 to 650 °C have been performed. The influence of catalyst weight/acetol flow rate (W/mAc) and steam to carbon molar (S/ C) ratios on gas yields has been analyzed. With the purpose of studying the influence of catalyst composition on gas yields, coprecipitated Ni-Al catalysts have been selected, some of them promoted with lanthanum and cobalt.

10.1021/ie060904e CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007

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Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007

Figure 1. Schematic of the experimental system.

2. Experimental Section 2.1. Experimental System. The experimental system is a bench-scale installation using a technology very similar to the Waterloo Fast Pyrolysis Process (WFPP).25 A schematic of the installation is shown in Figure 1. The fluidized-bed reactor is made of 316 stainless steel and the distributor plate of inconel. The body of the reactor has an inner section of 13.14 cm2. The reactor has a lateral arm for feeding liquids. This feeding system is composed of four concentric tubes. The aqueous solution of acetol is introduced by the inner tube. The second tube provides nitrogen for dispersing the acetol aqueous solution as a spray. The outer tubes are used as a cooling jacket. This reactor has been designed with the purpose of processing bio-oil or its fractions. Bio-oil has the inconvenience that it cannot be totally vaporized, and significant amounts of residual solids are often formed, blocking the feeding line and the reactor.26 The feeding system produces the appropriate dispersion of acetol aqueous solution at the feeding point, preventing thermal decomposition before the reaction bed is reached. The aqueous solution of acetol is delivered by a HPLC metering pump, AGILENT series 1100, with flow rates up to 5 mL/min. This solution is converted into gas once it reaches the bed because of the high temperatures encountered. The product gas is cleaned of solid particles that can be elutriated from the bed using a cyclone. The water and acetol that have not reacted together with any other liquid that may be formed are retained in a system of two condensers and a cotton filter. The cotton filter can also retain small solid particles of catalyst or carbonaceous residues. The CO and CO2 concentrations of the exit gas are continuously determined by an infrared analyzer. An AGILENT P200 Micro GC, equipped with TC detectors, is used to measure concentrations of H2, N2, CO, CO2, CH4, and C2 (C2H2, C2H4, and C2H6) in the product gas. The time required for the analysis is 3.2 min. The gas flow rates (N2, H2, and air) and CO and CO2 concentrations in the exit gas are registered by a data acquisition system.

The experimental system operated at atmospheric pressure. The majority of the experiments were performed using a concentration of acetol in the aqueous solution of 23% weight, corresponding to a steam to carbon molar (S/C) ratio of 4.6. The total nitrogen flow rate into the reactor was 2100 (STP) cm3/min. The noncatalytic steam reforming experiments were performed from 450 to 650 °C, while the catalytic steam reforming experiments were carried out at 600 and 650 °C. The reaction bed was composed of sand (264 g) and catalyst. The particle sizes of catalyst and sand used were between 160 and 320 µm. The catalyst weight, W, ranged from 0 in the noncatalytic experiments to 3 g. The inlet flow rate of the acetol aqueous solution, mAc, was 1.53 g/min. In the catalytic experiments, the catalyst weight/acetol flow rate ratio, W/mAc, was varied from 2.27 to 8.52 g catalyst min/g acetol, corresponding to values of GC1HSV from 22323 to 5947 h-1. In order to analyze the influence of the S/C ratio, some experiments were performed using 1.2 g of catalyst and acetol concentrations in the aqueous solution ranging from 18.6 to 50.7%, which corresponds to S/C molar ratios from 6 to 1.3. The W/mAc ratio was varied from 1.51 to 4.22 g catalyst min/g acetol. In these experiments, the total liquid flow rate and nitrogen flow rate were not modified with the purpose of obtaining similar fluidization conditions and the same total inlet flow rate. 2.2. Catalysts. The Ni-Al catalyst, Ni-Al catalysts promoted with lanthanum (Ni-Al-La), and Ni-Al catalysts promoted with cobalt (Ni-Co-Al) were prepared in the laboratory by coprecipitation. The preparation method was similar to that described by Al-Ubaid and Wolf.27 The Ni-Al catalyst with a Ni:Al atomic ratio of 1:2 was prepared by adding ammonium hydroxide to a solution of Ni(NO3)2·6H2O and Al(NO3)3·9H2O in distilled water until the pH reached 7.9. The precipitation medium was maintained at 40 °C and moderately stirred. The precipitate obtained was filtered and washed at 40 °C and dried for about 15 h at 105 °C. The precursor thus obtained was calcined for 3 h in air atmosphere at 750 °C. Three Ni-Al-La catalysts modified with 4, 8, and 12 wt.% of La2O3 in the

Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007 2401 Table 1. Results of Acetol Steam Reforming without Catalyst run 1

2

temperature (°C) 450 500 S/C (mol/mol) 4.6 4.6 liquid feeding rate (g/min) 1.53 1.53 time (min) 80 75 total acetol fed (g) 28.2 26.4 carbon conversion (%) 36.90 58.02 yields (g/g) total gas/(acetol + water) 0.094 0.155 total gas/acetol 0.410 0.675 gas yields (g/g acetol) H2 0.031 0.052 CH4 0.050 0.070 CO 0.283 0.439 CO2 0.032 0.096 C2 0.014 0.018 gas composition (% mol, N2- and H2O-free) H2 51.74 53.19 CH4 10.43 8.95 CO 33.74 32.08 CO2 2.43 4.46 C2 1.67 1.32

Figure 2. XRD spectra of calcined catalysts.

calcined catalyst were also prepared. In the case of the catalysts containing lanthanum, the final preparation pH was 8.05. The catalysts modified with lanthanum have the same nickel content as the Ni-Al catalyst, the lanthanum contained in the catalyst replacing aluminum. Two Ni-Co-Al catalysts were also prepared with Co/Ni atomic ratios of 0.25 and 0.025. The catalysts modified with cobalt have the same aluminum content as the Ni-Al catalyst, and a small amount of nickel has been replaced by cobalt. The preparation procedure was similar to that followed in the preparation of the Ni-Al catalyst. More details about the catalyst preparation can be found in previous works.20,24 The calcined catalyst was reduced in the reactor, just before the steam reforming reaction was performed, at a temperature of 650 °C during 1 h, using hydrogen diluted in nitrogen (H2: N2 ) 1:10). The hydrogen flow rate was 200 cm3(STP)/min. The calcined catalysts were characterized by X-ray diffraction (XRD) and nitrogen adsorption. Figure 2 shows XRD results for the six calcined catalysts used. NiO and NiAl2O4 crystalline phases were identified by XRD. Lanthanum phases were not detected, possibly due to the low proportion of this metal. Ni-Co-Al catalysts can also contain CoAl2O4 spinel. For the majority of catalysts the crystallinity was low with wide and asymmetric peaks. The sample with the highest crystallinity was the Ni-Co-Al catalyst with a Co/Ni atomic ratio of 0.25 because the peaks are narrower. It is also observed that the catalysts containing lanthanum have more NiO phase than the rest due to the signals at 2θ ) 43.3 ° having more intensity. The surface area of the Ni-Al catalyst was 150 m2/g, and for the catalysts modified with lanthanum, the surface areas were 149, 141, and 131 m2/g for 4, 8, and 12 wt % La2O3, respectively. It is appreciated that the addition of lanthanum causes a slight decrease in surface area. For Ni-Co-Al catalysts, the surface area was 152 and 131 m2/g for Co/Ni

3

4

5

550 4.6 1.53 83 29.2 75.96

600 4.6 1.53 66 23.2 77.57

650 4.6 1.53 65 22.9 87.09

0.196 0.850

0.195 0.849

0.220 0.956

0.066 0.107 0.544 0.099 0.034

0.069 0.115 0.528 0.090 0.047

0.065 0.127 0.609 0.112 0.043

52.73 10.69 31.05 3.60 1.94

53.68 11.18 29.34 3.18 2.61

49.04 11.98 32.82 3.84 2.32

atomic ratios of 0.025 and 0.25, respectively. It is also observed that the increase in cobalt content causes a decrease in surface area, which is in accordance with a sample with higher crystallinity. Further details about catalyst characterization can be found in previous works.20,24,28 2.3. Chemicals. The model compound selected for the tests, acetol, was supplied by SIGMA-Aldrich (99.2% purity). Other chemicals used in this study included commercial high purity gases, hydrogen, nitrogen, air, helium, and argon as well as standard gas mixtures (CO, CO2, and nitrogen) for calibration of the CO-CO2 analyzer and (H2, N2, CO, CO2, CH4, C2H2, C2H4, C2H6) for the calibration of the gas chromatograph. 3. Results and Discussion 3.1. Noncatalytic Steam Reforming. The purpose of these experiments was to study the influence of temperature in the noncatalytic steam reforming of acetol. A comparison of the results obtained in these tests with those of the catalytic steam reforming enables identification of the specific role of the catalyst in the process. In these experiments the reaction bed was composed of 264 g of sand. Table 1 presents the overall results obtained in experiments performed at different temperatures. The table shows the values of some experimental variables such as temperature, S/C molar ratio, liquid feeding rate, time, and total acetol fed. Also indicated are the percentage of carbon contained in the acetol converted into gases (CO, CO2, CH4, and C2), the yields of total gas expressed as mass fractions of the sum of acetol and water and acetol alone, the yields of different gases as mass fractions of acetol, and the gas composition expressed as molar percentages (N2 and H2O free). From the results it can be observed that when the temperature increases, carbon conversion to gas and total gas yields increase. The general tendency of H2, CH4, CO, CO2, and C2 yields show an increase in gas yields when the temperature increases. Gas composition does not change significantly with temperature; H2 content is around 52%, CH4 content around 10.5%, CO around 32%, CO2 content around 3.5%, and C2 around 2%. Unlike the result obtained in noncatalytic steam reforming of acetic acid14 at 650 °C, acetol carbon conversion to gas does not reach 100%. This fact indicates that acetol is a more difficult compound to convert than acetic acid.

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Table 2. Gas Yields for Equilibrium of Steam Reforming, Pyrolysis, and Experimental Noncatalytic Steam Reforming

Table 3. Results of Acetol Catalytic Steam Reforming: Influence of W/mAc Ratio at 650 °C Using Ni-Al Catalyst

yield (g/g acetol) temp (°C)

equilibrium steam reforming

450 500 550 600 650

0.122 0.154 0.170 0.172 0.171

450 500 550 600 650

0.044 0.095 0.155 0.207 0.254

run

equilibrium pyrolysis

expt

0.005 0.011 0.020 0.025 0.027

0.031 0.052 0.066 0.069 0.065

0.448 0.550 0.664 0.732 0.752

0.283 0.439 0.544 0.528 0.609

0.240 0.158 0.069 0.017 0.003

0.032 0.096 0.099 0.090 0.112

0.305 0.277 0.244 0.224 0.218

0.050 0.070 0.107 0.115 0.127

1 1 1 1 1

0.410 0.675 0.850 0.849 0.956

H2

CO

CO2 450 500 550 600 650

1.363 1.479 1.496 1.448 1.381 CH4

450 500 550 600 650

0.128 0.056 0.016 0.004 0.001

450 500 550 600 650

1.657 1.784 1.837 1.831 1.807

total gas

Table 2 presents a comparison of thermodynamic equilibrium yields for pyrolysis and steam reforming and experimental yields. Because carbon conversion to gas is lower than 100% at 650 °C and at this temperature the total gas yield (total gas/acetol) is close to 1, it appears that steam participates in the noncatalytic process. The comparison of experimental gas yields with equilibrium yields of steam reforming and pyrolysis indicates that most H2 and CO2 experimental yields are higher than pyrolysis equilibrium yields and lower than steam reforming equilibrium yields. Most CO and CH4 experimental yields are lower than pyrolysis equilibrium yields but higher than steam reforming equilibrium yields. These results also indicate some participation of steam in the noncatalytic process. 3.2. Influence of W/mAc Ratio. With the purpose of studying the influence of the W/mAc ratio, catalytic experiments have been performed at 600 and 650 °C. Tables 3 and 4 present the overall results obtained at 650 and 600 °C, respectively, and also include the results corresponding to thermodynamic equilibrium. From the results it can be observed that when the W/mAc ratio increases, carbon conversion to gas and total gas, H2, and CO2 yields increase, while CH4, CO, and C2 yields decrease. For the two temperatures a shift to thermodynamic equilibrium yields is observed with the presence of the catalyst and the increase of the W/mAc ratio. Figures 3-7 show gas yield evolution versus time for H2, CH4, CO, CO2, and total gas, respectively, at 650 °C. Figures 8 and 9 show the evolution of H2 and total gas yields versus time, respectively, at 600 °C. These figures also include the noncatalytic results and thermodynamic equilibrium yields. The tendencies observed in Tables 3 and 4 are corroborated in Figures 3-9.

temperature (°C) S/C (mol/mol) catalyst weight (g) W/mAc (g catalyst min/g acetol) space velocity, GC1HSV (h-1) liquid feeding rate (g/min) time (min) total acetol fed (g) carbon conversion (%) yields (g/g) total gas/(acetol + water) total gas/acetol gas yields (g/g acetol) H2 CH4 CO CO2 C2

6

7

8

9

650 4.6 0.8 2.27 22323 1.53 65 22.9 87.06

650 4.6 1.2 3.41 14860 1.53 65 22.9 87.22

650 4.6 2 5.68 8921 1.53 65 22.9 88.01

650 4.6 3 8.52 5947 1.53 65 22.9 93.38

equ

100

0.277 1.205

0.292 1.268

0.323 1.404

0.361 1.569

0.416 1.807

0.119 0.095 0.440 0.528 0.023

0.134 0.082 0.427 0.609 0.016

0.154 0.070 0.308 0.862 0.010

0.166 0.050 0.288 1.060 0.005

0.171 0.001 0.254 1.381 0

650 4.6

Table 4. Results of Acetol Catalytic Steam Reforming: Influence of W/mAc Ratio at 600 °C Using Ni-Al Catalyst run temperature (°C) S/C (mol/mol) catalyst weight (g) W/mAc (g catalyst min/g acetol) space velocity, GC1HSV (h-1) liquid feeding rate (g/min) time (min) total acetol fed (g) carbon conversion (%) yields (g/g) total gas/(acetol + water) total gas/acetol gas yields (g/g acetol) H2 CH4 CO CO2 C2

10

11

12

600 4.6 0.8 2.27 22323 1.53 60 21.11 81.19

600 4.6 1.0 2.84 17842 1.53 65 22.87 84.65

600 4.6 1.6 4.55 11137 1.53 65 22.87 89.59

equ

100

0.225 0.977

0.249 1.082

0.290 1.261

0.421 1.831

0.082 0.114 0.484 0.261 0.036

0.099 0.113 0.456 0.382 0.032

0.126 0.106 0.392 0.612 0.025

0.172 0.004 0.207 1.448 0

600 4.6

Figure 3 shows that experimental hydrogen yields at 650 °C using W/mAc ratios higher than 5.68 g catalyst min/g acetol reach thermodynamic equilibrium. In Figure 4, CH4 yields at 650 °C for the highest W/mAc ratio are not close to thermodynamic equilibrium, which could be explained by the slow reaction rate of methane steam reforming reaction at this temperature.

Figure 3. H2 yield evolution with time, influence of W/mAc ratio at 650 °C.

Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007 2403

Figure 4. CH4 yield evolution with time, influence of W/mAc ratio at 650 °C.

Figure 5. CO yield evolution with time, influence of W/mAc ratio at 650 °C.

The evolution of the results with the W/mAc ratio can be explained by the following reactions:

Figure 6. CO2 yield evolution with time, influence of W/mAc ratio at 650 °C.

Figure 7. Total gas yield evolution with time, influence of W/mAc ratio at 650 °C.

acetol steam reforming C3H6O2 + H2O f 3CO + 4H2

(1)

C3H6O2 + 4H2O f 3CO2 + 7H2

(2)

methane steam reforming CH4 + H2O T CO + 3H2

(3)

C2 steam reforming C2Hn + 2H2O f 2CO + (2 + n/2)H2

(4)

water-gas shift CO + H2O T CO2 + H2

(5)

The increase of carbon conversion can be explained by acetol steam reforming reactions. Methane steam reforming and C2 steam reforming reactions explain the decrease of CH4 and C2 yields, respectively. Water-gas shift reaction can explain the decrease of CO yield and the increase of CO2 yield. It can be concluded that the catalyst exerts an important effect in these reactions that incorporate water. Some of these reactions were also mentioned in the study of catalytic acetic acid steam reforming.14

Figure 8. H2 yield evolution with time, influence of W/mAc ratio at 600 °C.

3.3. Influence of S/C Molar Ratio. At 650 °C the influence of the S/C ratio on gas yields has been studied. Table 5 presents the overall experimental results. Runs 7, 14, and 17 show the effect of the S/C ratio for a W/mAc ratio close to 3.4 g catalyst min/g acetol. When the S/C ratio increases, carbon conversion to gas, total gas yield (expressed as mass fraction of acetol alone), and H2 and CO2 yields increase. The CO yield shows a

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Table 5. Results of Acetol Catalytic Steam Reforming: Influence of S/C Ratio at 650 °C Using Ni-Al Catalyst run temperature (˚C) S/C (mol/mol) catalyst weight (g) W/mAc (g catalyst min/g acetol) space velocity, GC1HSV (h-1) liquid feeding rate (g/min) time (min) total acetol fed (g) carbon conversion (%) yields (g/g) total gas/(acetol + water) total gas/acetol gas yields (g/g acetol) H2 CH4 CO CO2 C2

13

7

14

15

16

17

18

650 6 1.2 4.22 12008 1.53 65 18.46 93.50

650 4.6 1.2 3.41 14860 1.53 65 22.9 87.22

650 3.3 1.52 3.40 14904 1.54 65 29.06 86.68

650 3.3 1.2 2.68 18908 1.54 65 29.06 81.46

650 2 1.2 1.90 26670 1.55 65 41.02 75.24

650 1.3 2.7 3.40 14904 1.56 65 51.54 72.33

650 1.3 1.2 1.51 33558 1.56 65 51.54 75.88

0.256 1.379

0.292 1.268

0.350 1.208

0.337 1.161

0.429 1.057

0.518 1.022

0.510 1.007

0.134 0.093 0.384 0.749 0.019

0.134 0.082 0.427 0.609 0.016

0.113 0.099 0.415 0.562 0.019

0.112 0.082 0.396 0.555 0.016

0.094 0.074 0.388 0.488 0.013

0.095 0.060 0.407 0.448 0.012

0.090 0.080 0.474 0.351 0.012

slight increase when the S/C ratio increases. CH4 and C2 yields increase from a S/C ratio of 1.3 to 3.3 but decrease from a S/C ratio of 3.3 to 4.6. Figures 10 and 11 show the evolution of H2 and CH4 yields with time, respectively. The results of the experimental gas yields obtained when the S/C ratio varies can be explained by the participation of acetol steam reforming, water gas shift, and CH4 and C2 steam reforming reactions. The participation of water-gas shift and acetol steam reforming

Figure 9. Total gas yield evolution with time, influence of W/mAc ratio at 600 °C.

reactions can explain the increase of H2, CO, and CO2 yields. The decrease of CH4 and C2 yields when S/C ratio increases from 3.3 to 4.6 can be due to the participation of CH4 and C2 steam reforming reactions. Runs 13, 7, 15, 16, and 18 show the effect of W/mAc and S/C ratios taken together. When the W/mAc and S/C ratios increase, the general tendency is an increase in carbon conversion and total gas yield (expressed as mass fraction of acetol alone), H2 yield, and CO2 yield. The C2 yield increases when the S/C and W/mAc ratios increase. CO and CH4 yields do not show a clear tendency. The experimental results have been compared to those corresponding to thermodynamic equilibrium. The general tendency observed for experimental H2 and CO2 yields is also followed by equilibrium yields, although experimental yields are lower than equilibrium ones. Thus, the equilibrium H2 yield is 0.176 g H2/g acetol for a S/C ratio of 6. The equilibrium CO yield decreases when the S/C ratio increases, while the experimental CO yield appears quite constant with values around 0.4 g CO/g acetol. The equilibrium yields present values of 0.20 g CO/g acetol for a S/C ratio of 6 up to 0.61 g CO/g acetol for a S/C ratio of 1.3. Equilibrium CH4 yields decrease when the S/C ratio increases and equilibrium C2 yields are zero. Experimental CH4 and C2 yields are higher than equilibrium yields. The experimental results of CH4 yields differ more from the equilibrium data than C2 yields, which could mean that CH4 is more difficult to steam reform than C2 at 650 °C.

Figure 10. H2 yield evolution with time, influence of S/C ratio.

Figure 11. CH4 yield evolution with time, influence of S/C ratio.

Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007 2405 Table 6. Results of Acetol Catalytic Steam Reforming: Influence of Catalyst Composition at 650 °C run catalyst temperature (˚C) S/C (mol/mol) catalyst weight (g) W/mAc (g catalyst min/g acetol) space velocity, GC1HSV (h-1) liquid feeding rate (g/min) time (min) total acetol fed (g) carbon conversion (%) yields (g/g) total gas/(acetol + water) total gas/acetol gas yields (g/g acetol) H2 CH4 CO CO2 C2 a

7

19

20

21

22

23

Ni-Al 650 4.6 1.2 3.41 14860 1.53 65 22.9 87.22

Ni-Al-Laa 650 4.6 1.2 3.41 14860 1.53 65 22.9 81.58

Ni-Al-Lab 650 4.6 1.2 3.41 14860 1.53 65 22.9 95.18

Ni-Al-Lac 650 4.6 1.2 3.41 14860 1.53 65 22.9 92.75

Ni-Co-Ald 650 4.6 1.2 3.41 14860 1.53 65 22.9 82.96

Ni-Co-Ale 650 4.6 1.2 3.41 14860 1.53 65 22.9 82.22

equ

0.292 1.268

0.277 1.203

0.311 1.354

0.317 1.378

0.281 1.221

0.295 1.284

0.416 1.81

0.134 0.082 0.427 0.609 0.016

0.121 0.080 0.348 0.638 0.016

0.120 0.100 0.423 0.689 0.022

0.118 0.087 0.360 0.796 0.017

0.126 0.081 0.358 0.638 0.018

0.134 0.072 0.311 0.761 0.006

0.171 0.001 0.254 1.381 0

650

100

4 wt % La2O3. b 8 wt % La2O3. c 12 wt % La2O3. d Co/Ni ) 0.025. e Co/Ni ) 0.25.

3.4. Influence of Catalyst Composition. Table 6 shows the overall experimental results carried out with catalysts with different compositions. The catalysts selected were Ni-AlLa with 4, 8, and 12% of La2O3 in the calcined precursor and Ni-Co-Al catalysts with Co/Ni atomic ratios of 0.025 and 0.25. For the purposes of comparison, this table also includes experiment 7 carried out with the Ni-Al catalyst under the same experimental conditions and the data corresponding to thermodynamic equilibrium at 650 °C. The Ni-Al-La catalysts with high content of lanthanum (8 and 12% La2O3) present higher values of carbon conversion than the value of the Ni-Al catalyst, while the catalysts modified with cobalt present carbon conversion lower than the Ni-Al catalyst. The catalysts with the highest total gas yield are the NiAl-La catalysts with high contents of lanthanum. These catalysts also present the highest values of CH4, CO2, and C2 yields. All the catalysts modified with lanthanum present the lowest H2 yields. Some results are in accordance with acetic acid steam reforming with Ni-Al catalysts modified with lanthanum,14 such as the highest values of CH4 and C2 yields, and the lowest values of the hydrogen yield that decreases with the increase of lanthanum content. However, other results such as total gas and CO2 yields do not follow the same tendencies. The cobalt content in the catalyst significantly influences the gas yields. CH4 and CO yields decrease when the cobalt content increases, while the CO2 yield increases. These yields shift to thermodynamic equilibrium values when increasing the cobalt content. The catalyst with a Co/Ni atomic ratio of 0.25 presents the highest H2 yield together with the Ni-Al catalyst. 4. Conclusions Acetol has been selected as a model compound of bio-oil liquids derived from biomass pyrolysis. In this study, the influence of temperature on gas yields in the noncatalytic steam reforming of acetol has been analyzed. The influence of the W/mAc ratio on gas yields has been studied at 600 and 650 °C using a coprecipitated Ni-Al catalyst. The influence of the S/C ratio and catalyst composition on gas yields has also been investigated. For the latter purpose Ni-Al-La and Ni-CoAl catalysts have been selected. The main conclusions of this work are as follows:

(1) In the noncatalytic steam reforming of acetol, carbon conversion to gas, total gas yield, and yields to different gases increase when the reaction temperature increases. There appears to be some participation of steam in the noncatalytic process. (2) At 600 and 650 °C the presence of the catalyst in steam reforming of acetol increases H2, CO2, and total gas yields while CH4, CO, and C2 yields decrease. These evolutions of gas yields can be due to the participation of water gas shift, methane, and C2 steam reforming reactions. (3) The S/C molar ratio significantly influences acetol catalytic steam reforming at 650 °C. The increase of the S/C ratio increases H2, CO2, and total gas yields and carbon conversion to gas. (4) Ni-Al catalysts with high lanthanum content show the highest values of CH4, CO2, C2, and total gas yields and carbon conversion to gas, while also showing the lowest hydrogen yield. (5) Ni-Co-Al catalysts present lower carbon conversion to gas than the other catalysts tested. Cobalt content has a significant influence on gas yields. The catalyst with a Co/Ni atomic ratio of 0.25 presents the highest hydrogen yield obtained, similar to that of the Ni-Al catalyst. Acknowledgment The authors express their gratitude to the Gobierno de Aragoń (DGA) for providing financial support for the study (Project PIP185/2005). Literature Cited (1) Czernik, S.; French, R. J.; Magrini-Bair, K. A.; Chornet, E. The Production of Hydrogen by Steam Reforming of Trap Grease-Progress in Catalyst Performance. Energy Fuels 2004, 18, 1738. (2) Czernik, S.; French, R. J. Production of Hydrogen from Plastics by Pyrolysis and Catalytic Steam Reform. Energy Fuels 2006, 20, 754. (3) Czernik, S.; French, R.; Feik, C.; Chornet, E. Hydrogen by Catalytic Steam Reforming of Liquid Byproducts from Biomass Thermoconversion Processes. Ind. Eng. Chem. Res. 2002, 41, 4209. (4) Marquevich, M.; Coll, R.; Montane´, D. Steam Reforming of Sunflower Oil for Hydrogen Production. Ind. Eng. Chem. Res. 2000, 39, 2140. (5) Wang, D.; Czernik, S.; Montane´, D.; Mann, M.; Chornet, E. Biomass to Hydrogen via Fast Pyrolysis and Catalytic Steam Reforming of the Pyrolysis Oil or Its Fractions. Ind. Eng. Chem. Res. 1997, 36, 1507. (6) Wang, D.; Czernik, S.; Chornet, E. Production of Hydrogen from Biomass by Catalytic Steam Reforming of Fast Pyrolysis Oils. Energy Fuels 1998, 12, 19.

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ReceiVed for reView July 13, 2006 ReVised manuscript receiVed January 11, 2007 Accepted January 18, 2007 IE060904E

Hydrogen Production by Catalytic Steam Reforming of Acetol, a Model

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