Reaction Mechanism of Tar Evolution in Biomass Steam Gasification for Hydrogen Production

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1 Reaction Mechanism of Evolution in Biomass Steam ification for Hydrogen Production Shingo Katayama a, Masahiro Suzuki a, and Atsushi Tsutsumi a a Department of Chemical System Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, , Japan, tsutsumi@chemsys.t.u-tokyo.ac.jp (A. Tsutsumi) ABSTRACT: Reaction mechanism of evolution in steam gasification of biomass was investigated with a continuous cross-flow moving bed type differential reactor, in which and gases can be fractionated according to reaction time. We estimated that time profile of and gas evolution in the gasification of cellulose, xylan, and lignin, and compared it with experimental product time profile of real biomass gasification. The experimental evolution rate is different from estimated evolution rate. The estimated evolution rate has a peak at 20 s. On the other hand, the experimental evolution rate at 20 s is little, and at initial stage includes more water-soluble and water-insoluble compounds. It can be concluded that in the real biomass steam gasification the evolution of from cellulose and lignin component was found to be precipitated by that from hemicellulose component. KEYWORDS: Biomass gasification,, Continuous Cross-flow moving bed type Differential Reactor Introduction The steam gasification of biomass is a promising technology for thermochemical hydrogen production from biomass. Since biomass is a renewable and clean energy resource, biomass hydrogen is essential for the sustainable hydrogen society. In addition, among biomass conversion processes, gasification is one of the most efficient ones [1]. On the other hand, biomass gasification has problems such as production. is a complex mixture of condensable hydrocarbons such as aromatics. In biomass gasification process, a large amount of is produced, causing some trouble such as pipeline plugging, defluidization, and the reduction of thermal efficiency [2]. Thus, it is essential to elucidate the reaction mechanism of production in gasification for improvement in controllability and thermal efficiency of biomass gasifiers. So far, many researchers have conducted the experimental studies about pyrolysis of biomass and its components [3-30]. Especially, cellulose has most been studied. Broido- Shafizadeh model (Fig. 1) was the initially proposed mechanism for the pyrolysis of cellulose [3-5]. This model consists of three coupled first order reactions. The first stage of pyrolysis involves no weight loss and leads to form intermediate called active cellulose. This is followed by a pair of competing reactions which produces either volatiles or char and gases with weight loss. This concept of active cellulose was inherited in many pyrolysis or gasification models of cellulose proposed later [6-11, 13,14]. Moreover, extensive efforts for intermediate and evolution had been reported [11-14, 16-20]. Boutin et al. [17-20] observed that the active cellulose is liquid at the reaction temperature but solid at ambient temperature. The active cellulose is completely soluble in water (anhydro-oligosaccharides) and its calculated mean lifetime is 0.02 s at the reaction temperature [18]. Wooten et al. [16] found that final carbohydrate converts to aromatic carbons but not to aliphatic carbons when subjected to prolonged heating. With regard to the pyrolysis of real biomass, many researchers has been studied and proposed reaction models [23-27]. Most of those assume that cellulose, hemicellulose, and lignin are pyrolyzed independently. Müller-Hagedorn et al. [23] proposed a parallel reaction model of 5 components (cellulose 1, cellulose 2, hemicellulose 1, hemicellulose 2, and lignin) and evaluated formal kinetic parameters. Miller and Bellan [24, 25] considered that kinetic scheme of cellulose pyrolysis is also adopted to hemicellulose and lignin (Fig. 2). They mentioned that pyrolysis of biomass can be explained based on a superposition of the three components. 1/8 Cellulose k i k v Active cellulose k c Volatile Char + es Fig. 1 Broido-Shafizadeh model K 1 K 2 K 3 Virgin Active Cellulose Lignin or Hemicellulose K 4 x Char + (1-x) Fig. 2 Miller-Bellan model

2 However, in the conventional reactors for kinetic study of biomass gasification such as a thermobalance reactor, a drop tube reactor, a fixed bed reactor, and a fluidized bed reactor, it is impossible to investigate the time profile of and gas evolution during biomass gasification with differential method of kinetic analysis in continuous feeding condition. Thus, we have developed a continuous cross-flow moving bed type differential reactor (CCDR), in which biomass sample is continuously fed, and the products (, gas and char) can be fractionated from each compartment according to the reaction time. By using this apparatus, we previously researched the mechanism of pyrolysis of cellulose, and found that the dehydration of nascent char takes place after the devolatilization of is completed [8]. In the present study, the time profile of and gas evolution was investigated in steam gasification of biomass and its components: cellulose, xylan as hemicellulose, and lignin. The reaction temperature was kept at 673 K. The fractionated volatile matter and char were separated immediately and quenched. In this experimental condition, no secondary reaction between volatile matter and char takes place because of low temperature and short residence time. Based on the experimental results, we discuss on the reaction mechanism of evolution in biomass steam gasification. Experimental Apparatus Figure 3 shows the schematic diagram of a continuous cross-flow moving bed type differential reactor (CCDR). The reactor consists of a quartz glass half tube covered with a quartz glass plate and a belt conveyor system. The reactor is divided into six compartments (W90 mm D80 mm H40 mm) where gas flows are independent. The reactor is heated using an infrared gold image furnace (Ulvac Riko, Co. Ltd.). The heating zone of the furnace is divided into a preheat zone and three reaction zones. The temperatures of each zone are measured by K-type thermocouples and are controlled to be constant at 673 K independently. Biomass sample is fed out of a feeder onto the conveyor belt made of siliglass (SiO2 > 96%), which carries the sample across the six compartments, delivering produced char to a char sampling system. The initial time is defined as the time when sample is fed into the preheat zone. and gases produced in each compartment are sampled with a carrier gas and fractionated according to reaction time. At the end of belt conveyor, char is collected using the char sampling system. By changing the belt speed, the residence time of each compartment can be varied in the range of s. A bowl feeder (model PEF-90AL; Sanki Co.) included in an acrylic feeder box was used to feed biomass Fig. 3 Experimental apparutus : continuous cross-flow 2/8 moving bed type differential reactor

3 sample continuously with an Ar carrier gas into the preheat zone through a SUS316 tube (6.35 mm, o.d.). The feed rate was calibrated by weighing fed sample within 1 min before and after experiment, and calculate the average feed rate using both weight of fed sample. Steam was generated in a steam generator and was fed into the reactor with an Ar carrier gas. evolved in each compartment went through heated sampling lines, and then was collected separately in six traps that were cooled by ice. Water was also eliminated in CaCl 2 columns. Although the six sampling lines were gathered in a gas sampler (EMT 4SC6MWE; Valco Instruments Co.), only one of the lines was connected to a micro gas chromatograph (M-200H; Hewlett Packard Co.), and the line to the micro GC was changeable. The flow rate of the effluent gas was measured with a mass flow meter. By using the micro GC we analyzed H 2, O 2, N 2, CH 4 and with an MS-5A column and Ar carrier gas; 2, C 2 H 4 and C 2 H 6 with a Pora Plot Q column and Ar carrier gas. Procedure In this study, these are used as samples: Avicel microcrystalline cellulose (Merck Co. Ltd., < 160 μm, Avicel), xylan from birch wood (Sigma- Aldrich Co.), kraft lignin (Kanto Chemical Co. Ltd., μm) and real biomass (Chilean eucalyptus). The elemental compositions of these samples are shown in Table 1. The sample was dried at 333 K in an oven for 12 h prior to the experiment. The sample was put in the feeder and the feeder box was purged with Ar. The reactor was also purged with Ar until the concentration of O 2 declined below 1% and the flow rate of the carrier gas at each compartment was set to 400 cm 3 min -1. The steam generator, steam feeding lines and sampling lines were heated at 523, 453 and 423 K, respectively, to avoid the condensation of steam and. After heating the reactor up to 673 K, steam (40 vol. %) and cellulose were fed for 60 min. The calibration of the sample feed rate from the feeder was carried out at room temperature. The sample feed rate was kept at mg min -1 in this experiment. The belt speed was 750 mm min -1 and the reaction time of biomass sample was 38.4 s. Thus, the corresponding residence time of biomass sample in each compartment was 6.4 s. Analysis of After the experiment, distilled water was introduced into the reactor and sampling lines. Deposited was collected in the traps until the color of the solution became transparent. Acetone was also used as a solvent. Molecular weight of was also measured by a gel filtration chromatograph (pump: Waters 600 E Multi Solvent Delivery System, column: Shodex AsahipakGS-220HQ, detector: Waters 2414 differential refractometer) while KCl was added to the solution in order to reduce the intramolecular ionic repulsion between hydroxyl groups. After evaporating solvent in the solution, the composition of dried was analyzed with a CHNS elemental analyzer (2400II CHNS/O; Perkin Elmer). A small amount of the dried was milled with KBr (0.1 g) and measured by FT-IR (FTIR-230; JAS). The or cm -1 wave number rage with 16 scans was investigated. In each compartment of CCDR the evolution rates of gas and were defined as the follows: 100 M c evolution rate ( carbon molar basis) = M τ 80 M G evolution rate ( weight basis) = 60 W τ where M c, M, τ, M G, and W represent a molar amount of carbon in product in one compartment [mol], total molar carbon in fed biomass [mol], residence time in one compartment [s], molar amount of gases in product in one compartment [mol], and total amount of fed biomass [g], respectively. Results and discussion Trend of gas and evolution in gasification of each sample Figure 4 shows the conversion rate to the gas, and char in steam gasification of cellulose, lignin, xylan and real biomass. The estimated biomass carbon conversion as a superposition of cellulose (50 %), lignin (27 %), and xylan (23 %) is also shown in Fig. 4. This ratio of three main Table 1 Elemental compositions of samples (wt %) C H N O S Cellulose Lignin Xylan Real Biomass 3/8 Carbon Conversion Rate [%] cellulose lignin xylan uncollected gas biomass char superposition Fig. 4 Carbon conversion rate of each sample

4 components of biomass is estimated with elemental analysis of biomass by the measure reported by Hasegawa et al. [31]. Figures 5-8 show the time profile of evolution rate of, 2 (weight basis) and (carbon molar basis) during steam gasification of these samples at 673 K for cellulose. Figure 9 shows the evolution rate estimated as a superposition of cellulose, lignin, and xylan. In these figures, abscissa represents the elapsed time after the sample is fed onto the quartz belt. Since the gas evolution rate of H 2, CH 4, C 2 H 4, and C 2 H 6 was smaller than 2% of that of 2 in any case, these gases are not represented in the figures. In case of cellulose (Fig. 4), most of volatile was observed to be water soluble. It cam be seen in Fig. 5 that the evolution of increased rapidly with reaction time to achieve a peak at 22.4 s. In addition, the evolution of gas was found to exhibit a peak at 28.8 s. These results are attributed to the existence of intermediates [8]. In case of lignin, about 70 % of sample converted to char, and the amount of was very small as shown in Fig. 4. It was see from Fig. 6 that the initial stage exhibits large evolution rates of and gas. With increasing time the evolution rates of and gas decreased. Most of was water insoluble. This was attributed to the composition of lignin which 8 6 mainly consists of hydrophobic aromatic components. For xylan, the amount of gas was observed to 2 be larger than that of cellulose and lignin (Fig. 4) In Fig. 7 the time profile of gas evolution for xylan gasification was found to exhibit two peaks at the initial stage and 28.8 s. This result corresponds to the report by Müller-Hagedorn et Fig. 5 Time profile of gas and evolution (cellulose) Evolution Rate Fig. 6 Time profile of gas and evolution (lignin) Evolution Rate Evolution Rate [mol-c in / mol-c in sample s -1 ] Evolution Rate Fig. 7 Time profile of gas and evolution (xylan) Evolution Rate [mol-c in / mol-c in sample s -1 ] Evolution Rate [mol-c in / mol-c in sample s -1 ] Evolution Rate Evolution Rate [mol-c in / mol-c in sample s -1 ] Evolution Rate Evolution Rate [mol-c in / mol-c in sample s -1 ] Fig. 8 Time profile of gas and evolution (biomass : experimental result) Fig. 9 Time profile of gas and evolution (biomass : estimated as superposition of cellulose (50 %), lignin (27 %), and xylan (23 %)) 4/8

5 The Amount of [g / g-sample] Water Insoluble Water Soluble (a) al. [23] which shows hemicellulose has two decomposition steps. The dominant evolution takes place at initial stage. The amount of in xylan gasification was smaller than that in cellulose gasification and larger than that in lignin gasification. In case of biomass (Fig. 8), a similar trend for and gas evolution was observed with xylan gasification. The and gas evolution was found to exhibit two peaks at the initial stage and s. As can be seen in Fig. 9, the estimated time profile of gas evolution was similar to that of experimental data. The gas evolution exhibits two peaks at the initial stage and 28.8 s. In spite of similar trend for gas evolution, evolution was found to exhibit a different manor. In the initial stage of the large real biomass gasification, evolution rate was large and reduced with increasing time to a minimum at 22.4 s. On the other hand, evolution was found to be estimated to increase with time to reach a maximum at 22.4 s. Figures 10 (a) and 10 (b) show the evolution time profiles of water-insoluble and water soluble for the experimental result and the estimation by means of the superposition of cellulose, lignin and xylan, respectively. In the experimental result, both watersoluble and water-insoluble evolved at initial stage, and then the ratio of water-insoluble decreased with reaction time. On the other hand, in the estimation of evolution most of was watersoluble because of small amount of from lignin and xylan, and most of evolved from cellulose was water soluble. These results suggest that, in biomass gasification, the amount of water-insoluble which mainly evolves from lignin increases and the evolution of water soluble which evolves from cellulose occurs earlier than in the cellulose gasification. Analysis of Figure 11 shows FT-IR spectra of water-soluble from cellulose, xylan, and biomass (3.2 s and 28.8 s). No significant change in FT-IR spectra of evolved from cellulose and xylan was observed with reaction time. from cellulose has some peaks at cm -1. These are considered to be assigned to α1-6 bond of levoglucosan, the main component of water-soluble from cellulose. from xylan has a strong 5/8 The Amount of [g / g-sample] Water Insoluble Water Soluble Fig. 10 Amount of in biomass gasification (a) experiment (b) estimated as the superposition of cellulose, lignin, and xylan Transmittance [a. u.] from cellulose from xylan from biomass (3.2 s) (b) from biomass (28.8 s) Wave number [cm -1 ] Fig. 11 FT-IR spectra of produced water-soluble from cellulose, xylan, and biomass (reaction time : 3.2 s and 28.8 s)

6 peak at about 1400 cm -1. The from biomass gasification at 3.2 s after the beginning of reaction also has a comparatively strong peak at 1400 cm -1. derived from biomass at 28.8 s has some distinct peaks at cm -1. It is concluded that at initial stage in biomass gasification is mainly from xylan and at about 30 s later, derived from cellulose increases. The FT-IR spectra of lignin and water-insoluble from biomass (3.2 s and 28.8 s) were shown in Fig. 12. The spectrum of lignin shows peaks assigned to aromatic ring at 1600, 1510, 1420 cm -1. from biomass at 3.2 s also has these peaks. Figure 13 shows the molecular weight distribution of water-insoluble from biomass (reaction time: 3.2 s and 28.8 s) measured by the gel filtration chromatography (GFC). Hasegasa et al. reported that the molecular weight distribution of derived from cellulose has peaks at about 150 and 300, and the MW of from lignin is distributed at about The MW distribution of at 3.2 s of reaction time has a broad peak at This suggests that the water-insoluble at initial stage of biomass gasification contains the component derived from lignin. Scheme of biomass gasification From the analyses of, the evolution mechanism of and gas in biomass gasification is summarized schematically in Fig. 14. Horizontal axis represents elapsed time from the beginning of gasification, and vertical axis shows the amount of produced gas and. In cellulose gasification, and gas evolution rate increases with reaction time. The evolutions of and gas achieve a peak at about 20 s and 30 s, respectively. On the other hand, in biomass gasification, evolution of water-soluble showed peak early in the reaction. Thus, it can be considered that cellulose component of biomass releases earlier than pure cellulose. Pure lignin releases gas at initial stage of gasification. With little evolution, most of lignin converted into char. In biomass gasification, however, at initial stage water-insoluble was produced much more than expected from the gasification of lignin alone. As this contained aromatic components derived from lignin, it is considered that the evolution of from lignin component of biomass was enhanced comparing with pure lignin gasification. WHEC 16 / June 2006 Lyon France Transmittance [a. u.] Lignin (sample) from biomass (3.2 s) from biomass (28.8 s) Wave number [cm -1 ] Fig. 12 FT-IR spectra of lignin and produced water-insoluble from biomass (reaction time : 3.2 s and 28.8 s) Large MW MW s Retention time Small MW Fig. 13 Molecular Weight distribution of waterinsoluble measured by GFC In case of xylan, the representative of hemicellulose, the evolution of gas shows two peaks at initial stage and 30 s later. evolution occurs at the beginning of the reaction, and the amount is more than lignin but much less than cellulose. As mentioned above, evolution of biomass gasification cannot be estimated by the superposition of cellulose, lignin, and hemicellulose. The increase in derived from lignin component in real biomass gasification is likely to be due to the effect of reaction of hemicellulose. Omori et al. [32] indicated that holocelluose (hemicellulose and α-hemilose) acts as hydrogen donor. Sagehashi et al. [33] reported that in biomass (Japanese cedar) gasification production of phenol and guaiacol surpassed the amounts predicted from the result of cellulose, xylan, and lignin. They considered that holocelluose which acts as hydrogen donor accelerates the pyrolysis of biomass. On the other hand, Müller-Hagedorn et al. [23] reported that inorganic salts promote pyrolysis reaction. In addition, some researchers have reported the catalytic effect of 3.2 s 6/8

7 alkali and alkaline earth metallic (AAEM) species for biomass gasification [34, 35]. Thus, it is also likely that the inorganic salts like AAEM influences the evolution. Further investigation is needed to reveal the factor which influences the evolution of in biomass gasification. Conclusions By using a continuous cross-flow moving bed type differential reactor (CCDR), which can separate the volatiles according to reaction time, steam gasification of cellulose, lignin, xylan, and real biomass was investigated at 673 K. The fractionated was measured by weight and analyzed by FT-IR spectrometry, gel filtration chromatography, and elemental analysis. It is proven that in real biomass gasification the evolution rate of gas is similar to that of the predicted as superposition of cellulose, lignin, and xylan. On the other hand, the experimental evolution rate of real biomass is different from estimated evolution rate from each gasification result of the three components. The estimated evolution rate has a peak at 20 s, but experimental evolution rate has two peaks at the initial stage and 30 s. derived from real biomass at initial stage includes more water-soluble and water-insoluble compounds. From the analyses of, it is indicated that the composition of varies with reaction time. at initial stage of gasification of real biomass includes both water-soluble derived from cellulose and hemicellulose, and water-insoluble from lignin. In addition, the main component of at 30 s of reaction time is water-soluble from cellulose. Acknowledgement This study was financially supported by a Core Research for Evolutional science and Technology (CREST) grant from the Japan WHEC 16 / June 2006 Lyon France (water soluble) Science and Technology Agency (JST) and New Energy and Industrial Technology Development Organization (NEDO). The authors appreciate this financial support. (in biomass, a part of is released earlier) Cellulose (water insoluble, including aromatics) (in biomass, the amount of increases) Lignin (water soluble and insoluble, not including aromatics) Hemicellulose from cellulose from hemicellulose from lignin Biomass Fig. 14 Schematic diagrams for evolution of gas and in biomass gasification at 673 K References: 1. Bridgewater, A. V. The technical and economic feasibility of biomass gasification for power generation. Fuel, 74, , Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. A review of the primary measures for elimination in biomass gasification processes. Biomass and Bioenergy, 24, , Shafizadeh, F; Lai, Y.Z. Thermal Degradation of 1,6-Anhydro-p-β-glucopyranose, J. Org. Chem. 37, , Bradbury, A. G. W.; Sakai, Y.; Shafizadeh, F. A kinetic model for pyrolysis of cellulose. J. Applied Polymer Science, 23, , Shafizadeh, F. J. Introduction to pyrolysis of biomass. J. Anal. Appl. Pyrolysis, 3, , Fushimi, C.; Araki, K.; Yamaguchi, Y.; Tsutsumi, A. Effect of Heating Rate on Steam ification of Biomass. 1. Reactivity of Char. Ind. Eng. Chem. Res., 42, , Fushimi, C.; Araki, K.; Yamaguchi, Y.; Tsutsumi, A. Effect of Heating Rate on Steam ification of Biomass. 2. Thermogravimetric-Mass Spectrometric (TG-MS) Analysis of Evolution. Ind. Eng. Chem. Res., 42, , /8

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