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X-ray CT visualization of woody char intra-particle pore structure and its role on anisotropic evolution during char gasification Hirotatsu Watanabe Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03227 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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X-ray CT visualization of woody char intra-particle pore structure and its role on anisotropic evolution during char gasification Hirotatsu Watanabe1* Department of Mechanical Engineering, School of Engineering, Tokyo Institute of Technology, 2-12-1-NE-6, Ookayama, Meguro-ku, Tokyo 152-8550 Japan, Tel: +81-3-5734-2179, Fax: +81-3-5734-2179, Email: [email protected] KEYWORDS. X-ray CT, biomass, intra-particle structure, gasification

ABSTRACT. X-ray computed tomography (CT) was used to visualize the intra-particle structure of woody biomass and its chars. Additionally, the impact of intra-particle structure on gasification characteristics was investigated. The chars derived from ramin (hardwood) and Japanese cypress (softwood) were gasified under an O2/Ar atmosphere. X-ray computed tomography (CT) was used to observe intra-particle structure. As a result, X-ray CT performed well in visualizing relatively large pores of over 5 µm. The ramin chars had larger pores aligned along the transverse direction in comparison to Japanese cypress chars. The pores aligned along

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the grain direction were dominant in both chars. Although the gasification rate of both chars was similar, the impact of the anisotropic intra-particle structure originating from the nature of wood manifested in the evolution of particle shape during gasification controlled by internal diffusion. This was due to large pores acting as gas transport pathways inside the chars. The gasification advanced along the transverse direction in the ramin chars, and resulted in the formation of an ellipse shape, although the initial shape was a cylinder. On the contrary, in the Japanese cypress chars the gasification progressed only along the grain direction and the cylindrical shape was maintained.

1. Introduction Woody biomass is an important source of renewable energy [1]. Currently, biomass is mainly utilized as a substitute to fossil fuels in large (> 50MWth) efficient steam power plants that reach electric efficiencies of up to 40-50% [2]. However, in smaller typical biomass power plants (1050 MWth), the electrical efficiencies drop to 18-33% [2], whereas biomass has been recognized as an ideal energy resource for decentralized energy systems [3]. Thus, advanced biomass conversion with high efficiency is essential for future energy systems. Gasification is an efficient technology for extracting energy from woody biomass. The char gasification is a slower process than pyrolysis and volatile reactions [4,5]. Therefore, char reactivity is a key factor for enhancing gasification. Studies of char reactivity are summarized in a recent review [6]. In addition to the gasification, char has been receiving an increasing amount of attention with regard to utilization in direct carbon fuel cells (DCFCs) [7]. DCFC can convert bio-char into electricity without the need for gasification [8-13]. Although the development of this technology has been relatively slow, a recent review has pointed out that the field of DCFC has been attracting an ever


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increasing amount of interest [13]. Since the char has a potential role in different reaction processes such as gasification and DCFC, as described above, the chemical and physical characterization of chars has become important. Some research groups have studied the effect of particle size on thermochemical conversion, such as pyrolysis, and have pointed out the importance of intra-particle phenomena inside the particle [14-16]. Intra-particle phenomena are directly linked to the gas transport processes inside the char; therefore, intra-particle structure plays an important role in various reaction processes. Recent technical advances of X-ray CT have allowed submicron resolution, and this has resulted in increased challenges with regard to capturing the range of structural information in coal samples [17]. Our previous study visualized the anisotropic intra-particle structure of Japanese cypress chars, and the impact of anisotropic structure on char gasification has been investigated [18,19]. The intra-particle structure of softwood, such as Japanese cypress is supposed to be different to that of hardwood. The difference of hardwood and softwood has been discussed with regard to thermochemical processes from the viewpoint of chemical composition, and with consideration to the fact of hardwoods having a higher proportion of cellulose and hemicelluloses in comparison to softwoods. However, softwoods have a higher proportion of lignin [20-22]. Moreover, a few studies have investigated the difference of intra-particle structure and its impact on char gasification. A recent study has pointed out that intra-particle heat and mass transfer in a bed of particles is required for a more detailed model description [22]. In this study, the intra-particle structure of raw woods and its chars were observed by using Xray CT. The impact of intra-particle structure on gasification was investigated experimentally using chars derived from two different wood cylinders (softwood and hardwood).


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2. Experimental section In this study, the wood samples used were ramin (hardwood) and Japanese cypress (softwood) wood cylinders. The ultimate analyses of these samples is presented in Table 1. Ash contents of ramin and Japanese cypress are 0.5 and 0.2 wt% (dry base), respectively. The wood cylinder had a diameter of 5 mm and length of 5 mm. Pyrolysis and gasification experiments were conducted by using a thermobalance (ULVAC, TGD-9600) under atmospheric pressure, as shown in Fig. 1. In this experiment, the top and side surface of the wood cylinder were exposed to the surrounding gas. Air inside the experimental setup was purged by a vacuum pump and then replaced by argon. First, wood cylinders (ramin and Japanese cypress) were pyrolyzed under an Ar atmosphere in order to produce chars. Our previous work showed that temperature gradient inside biomass clearly appeared during pyrolysis when heating rate was higher than 10 K/s [23]. In this study, low heating rate of 1 K/s was used to produce uniform temperature gradient during pyrolysis. The final temperature of pyrolysis was 1173 K with a holding time of 5 m. Then, the char was gasified under O2/Ar (20/80 vol%), at constant temperatures. The details of the experimental procedure were presented in our previous work [16,18,19]. The char conversion (X) and gasification rate (R) were calculated by using Eq. (1) and Eq. (2), respectively. m m0

(1)

1 dm m0 dt

(2)

X =1−

R=−

where m is the mass of char, and m0 is the initial mass of the char. Our previous study showed that in the temperature range of 873-1173 K, the gasification reaction of the Japanese cypress chars used in this study was controlled by internal diffusion process from Arrhenius plot [18].


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Ramin chars showed similar trends. Thus, the evolution of particle shape during gasification was investigated by using both chars at 1173 K, where char gasification was controlled by internal diffusion. In the present study, the char reactivity (Ri) was defined as the gasification rate at X = 0.5. When the char conversion reached 0.6, the char was rapidly quenched in order to observe the shape and intra-particle structure of chars. After partial gasification, the temperature of 1173 K was decreased to 873 K within 30 sec, and then decreasing to the room temperature by terminating heating. In addition, the gas line was switched to Ar flow to terminate a reaction. The char reactivities shown in this paper were average three repeated experiments, and representative observation images were shown. Outer ash layer was formed during Ramin pyrolysis. Before observation, outer ash layer, which crumbled easily, was removed by a brush in order to focus on carbon structure. X-ray CT was used to investigate the internal structure of raw biomass and char samples. A cylinder with a diameter of 2 mm and length of 2 mm in the char was visualized at a resolution of approximately 2 µm. X-ray CT visualization was performed at BL47XU at SPring-8 for raw wood samples. Xray CT facilities (Xradia VersaXRM-500 and Yamato Scientific Co. Ltd. TDM1000H-Sµ) were used to visualize chars derived from Japanese cypress and ramin, respectively. The Micromeritics Tristar II model 3020 apparatus was used to measure the specific surface area. Chars were outgassed for 2 h at 433 K under vacuum, prior to the gas adsorption experiments, in order to eliminate moisture and condensed volatiles that could prevent adsorbate accessibility. In this study, CO2 adsorption isotherms were performed at 273 K. For CO2, the ranges of relative pressure (P/P0) were 0.005–0.03. The Dubinin–Radushkevich (D–R) equation was applied to the adsorption data in order to estimate the specific surface area. In addition to D– R equations, the isotherms were analyzed by using the Micromeritics density functional theory


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(DFT) software package for CO2, in order to obtain the pore size distribution in the size range of 0.4–0.8 nm.

3. Results and discussion 3-1 X-ray CT visualization Fig. 2 shows the overall pictures and X-ray CT cross-sectional images of the raw wood cylinders ((a) ramin and (b) Japanese cypress) at the resolution of 1.74 µm/pixel. Many pores existed in the raw wood. Many pores (over 5 µm) were visualized adequately with X-ray CT. Large and small pores were observed in the ramin, whereas, uniform pores were observed in Japanese cypress. Especially, pores larger than 100 µm were only observed in ramin. Fig. 3 shows the 3D X-ray CT images of the initial chars at X = 0 derived from Japanese cypress and ramin. White blocks in the ramin char are likely to be ash, since the attenuation coefficient of ash was quite different to that of carbon. Thus, X-ray CT can be used to make up images, which provide direct information with regards to spatial distribution of composition [17]. The honeycomb-like structure of small pores was observed in the partially cross-sectional view (xy cross section). Large and small pores were still observed in the ramin char, whereas, uniform pores were observed in the Japanese cypress char, which indicates that the pore structure of raw wood cylinder, as shown in Fig. 2, was maintained after pyrolysis at 1 K/s. The 3D X-ray CT visualization showed that pores inside the char generally aligned along its grain direction (z), and that the intra-particle structure was anisotropic, as shown in our previous research [18,19]. An important finding here was that the ramin chars had larger pores aligned along the transverse direction, in comparison to the Japanese cypress chars.


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Fig. 4 shows the 2D X-ray CT images of the initial chars in the x-y, y-z, and x-z cross-sectional chars images. In the Japanese cypress chars, small pores (approximately 5 µm) were seen sparsely in the y-z cross-section, whereas, pores were hardly observed in the x-z cross-section. Meanwhile, larger pores than 20 µm were observed in the ramin chars in the y-z and x-z crosssection. Especially, pores in the x-z cross-section (along y direction ) were larger than those in the y-z cross-section (along x direction). Large pores acted as gas transport pathways; thus, gas permeability was anisotropic in both chars. For the ramin chars, it was expected that gas would be more easily permeated along the y direction than along the x direction. Fig. 5 shows the Raman spectra of the initial Japanese cypress and ramin chars. In general, char exhibits two strong peaks in the D (defect) band (approximately 1480 cm-1) and the G (graphite) band (approximately 1600 cm-1). The spectra of the ramin chars were very similar to the spectra of Japanese cypress chars. Thus, the difference of char oxidation between Japanese cypress and ramin was caused not by chemical structure of carbon. Physical structure, including anisotropic intra-particle structure as shown in Fig. 4, became significant when the char oxidation was controlled by diffusion process.

3-2 Evolution of char structure during gasification Fig. 6 shows Arrhenius plot of char reactivities for ramin and Japanese cypress chars. With increasing temperature, the rate-limiting process of the gasification changed from the chemical reaction to the internal diffusion. The decrease in slope of reactivity indicated a transition into the internal diffusion-controlled zone of char gasification. Our previous studies showed that the gasification reaction of the Japanese cypress char was controlled by internal diffusion in the temperature range of 873-1173 K [18]. The gasification reaction rates were measured in this


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study, and those of ramin chars were almost the same as those of the Japanese cypress chars when the reaction was controlled by internal diffusion. At 1173 K, char reactivity was similar, and the reaction was controlled by diffusion process in both chars. Fig. 7 shows photographs of raw wood, initial chars at X = 0 and chars at X = 0.6 that were gasified at 1173 K. The frames of initial chars are also shown, in the char photos, at X = 0.6. After pyrolysis, the chars shrunk. The outer shape of the Japanese cypress char was almost the same as that of the ramin chars, and the tops and bottoms of both chars were circular at X = 0. The particle densities and particle sizes were decreased during gasification. A notable difference was observed in the particle shape of the chars at X = 0.6. An elliptic form was observed in the bottom of the ramin char, whereas, the circular form was maintained in the Japanese cypress chars at X = 0.6. The degree of circularity (fcirc) was calculated by using following equation:

݂ୡ୧୰ୡ =

ସగ஺

(3)

௉మ

where A and P are the cross-sectional area and the perimeter of the bottom of the char, respectively. The degrees of circularities were 0.85 and 0.74 for the Japanese cypress and ramin chars, respectively, whereas, for both of the initial chars, they were close to 1.0. It is known that the permeability of gas along fibres is much higher than transverse direction due to the anisotropic properties of wood [24,25]. Thus, pores at micron scales visualized by X-ray CT used here likely played an important role in gas permeability. Anisotropic gas permeability is likely to have caused this difference in shape evolution during gasification. As discussed in Fig. 4, pores along the transverse direction in the Japanese cypress chars were so small that gas permeability was likely to be lower than that of the ramin chars, which had larger pores along the transverse direction. In the ramin chars, the gasification progressed along the transverse direction and grain direction, whereas, in the Japanese cypress chars, it progressed only along the grain


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direction. In the ramin chars, the pores along the y direction were larger than the pores along the x direction, which indicates that gas permeability was different between the x and y direction. This resulted in the formation of an elliptic shape during char gasification. However, further investigation is still necessary to correlate pore size with gas permeability. Anisotropic particle evolution is useful in calculating the particle aerodynamics in the numerical simulation of biomass combustion or gasification. Fig. 8 shows the 3D and 2D X-ray CT images of the ramin char gasified at 1173 K (X=0.6). The pore structure remained during gasification, and it is suggested that, during gasification, the pores played an important role in gas transport processes inside the chars. Fig. 9 shows the measured incremental surface area of chars at (a) X = 0 and (b) X = 0.6. Table 2 shows the specific surface area determined by the D-R method and DFT. Although the surface area increased after gasification due to pore development, an insignificant difference existed in both chars. The gas adsorption technique could determine the specific surface area and nanoscale pore size; however, it could not characterize the anisotropic structure of larger pores as visualized by X-ray CT.

4. Conclusion In this study, the impact of intra-particle structure on gasification characteristics was investigated by using chars derived from hardwood (Ramin) and softwood (Japanese cypress). X-ray CT performed well in the visualization of the intra-particle structure of raw biomass and its chars. The pore structure of the raw wood cylinder was preserved after heating. Although pore alignment along the grain direction was dominant in both chars, ramin derived chars had larger pores aligned along the transverse direction, in comparison to the chars derived from Japanese


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cypress. The impact of intra-particle structure originated from the nature of wood, and appeared not in the gasification rate but in the anisotropic evolution of particle shape during gasification. The gasification progressed along the transverse direction in the ramin chars, whereas, in the Japanese cypress chars, it progressed only along the grain direction. This was due to the anisotropic permeability having been derived from the nature of woody biomass.

ACKNOWLEDGMENTS This study was partly supported by IHI corporation. The synchrotron radiation experiments were performed at the BL47XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI).

References [1] Lauri, P., Havlík, P., Kindermann, G., Forsell, N., Böttcher, H., Obersteiner, M. Woody biomass energy potential in 2050. Energy Policy 2014;66:19-31. [2] Gadsbøll, R. Øa., Thomsen, J., Bang-Møller, C., Ahrenfeldt, J., Henriksen, U. B. Solid oxide fuel cells powered by biomass gasification for high efficiency power generation. Energy 2017;131:198-206.


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[3] Lee, U., Balu, E., Chung, J. N., An experimental evaluation of an integrated biomass gasification and power generation system for distributed power applications, Applied Energy 2013;101:699-708. [4] Link, S., Arvelakis, S., Hupa, M., Yrjas, P., Külaots A., Paist A. Reactivity of the biomass chars originating from reed, douglas fir, and pine. Energy & Fuels 2010;24:6533-6539. [5] Cetin, E., Gupta, R., Moghtaderi, B. Effect of pyrolysis pressure and heating rate on radiata pine char structure and apparent gasification reactivity, Fuel 2005;84:1328-1334. [6] N. Mahinpey, A. Gomez, Review of gasification fundamentals and new fndings: Reactors, feedstock, and kinetic studies, Chem. Eng. Sci. 2016;148:14-31. [7] Qian, K., Kumar, A., Zhang, H., Bellmer, D., Huhnke, R. Recent advances in utilization of biochar, Renewable and sustainable energy reviews 2015;42:1055-1064. [8] Cherepy, N. J., Krueger R., Fiet, K. J., Jankowski, A. F., Cooper, J. F. Direct conversion of carbon fuels in a molten carbonate fuel cell. J. Electrochem Soc. 2005;152:A80-A87 [9] Kacprzak, A., Koby1ecki, R., W1odarczyk, R., Bis, Z. The effect of fuel type on the performance of a direct carbon fuel cell with molten alkaline electrolyte. J. Power Sources 2014;255:179-186. [10] Elleuch, A., Boussetta, A., Yu, J., Halouani, K., Li, Y. Experimental investigation of direct carbon fuel cell fueled by almond shell biochar: Part I. Physico-chemical characterization of the biochar fuel and cell performance examination. Int. J. Hydrogen Energy 2013;38:1659016604. [11] Watanabe, H., Kimura, T., Okazaki, K. Impact of ternary carbonate composition on the morphology of the carbon/carbonate slurry and continuous power generation by direct carbon fuel cells. Energy & Fuels 2016;30:1835-1840


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[12] Watanabe, H., Kimura, A., Hanamura, K. Impact of gas products around the anode on the performance of a direct carbon fuel cell using a carbon/carbonate slurry. J. Power Sources 2016;15:567-573 [13] Jiang, C., Ma, J., Corre, G., Jain, S. L., Irvine, J. T. S. Challenges in developing direct carbon fuel cells. Chem. Soc. Rev. 2017;46:2889-2912. [14] Bennadji, H., Smith, K., Serapiglia, M. J., and Fisher, E. M. Effect of particle size on lowtemperature pyrolysis of woody biomass. Energy and Fuels 2014;28:7527-7537. [15] Westerhof, R. J. M., Nygard, H. S., van Swaaij, W. P. M., Kersten, S. R. A., Brilman, D. W. F. Effect of Particle Geometry and Microstructure on Fast Pyrolysis of Beech Wood, Energy and Fuels 2012;26:2274-2280. [16] Pattanotai, T., Watanabe, H., Okazaki, K. Experimental investigation of intraparticle secondary reactions of tar during wood pyrolysis. Fuel 2013;104:468-475. [17] Mathews, J. P., Campbell, Q. P., Xu, H., Halleck, P. A review of the application of X-ray computed tomography to the study of coal. Fuel 2017;209:10-24. [18] Pattanotai, T., Watanabe, H., Okazaki, K. Gasification characteristics of large wood chars with anisotropic structure. Fuel 2014;117:331-339 [19] Pattanotai, T., Watanabe, H., Okazaki, K. Effect of particle aspect on pyrolysis and gasification of anisotropic wood cylinder. Fuel 2015;150:162-168. [20] Effendi, A., Gerhauser, H., Bridgwater, A. V. Production of renewable phenolic resins by thermochemical conversion of biomass: A review. Renewable and sustainable energy reviews 2008;12:2092-2116.


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[21] Garcìa-Pérez, M., Chaala, A., Pakdel, H., Kretschmer, D., Roy, C., Vacuum pyrolysis of softwood and hardwood biomass Comparison between product yields and bio-oil properties, J. Anal. Appl. Pyrolysis 2007;78:104-116. [22] Anca-Couce A., Obernberger, I. Application of a detailed biomass pyrolysis kinetic scheme to hardwood and softwood torrefaction. Fuel 2016;167:158-167. [23] Okekunle, P. O., Watanabe, H., Pattanotai, T., Okazaki, K., J. Therm. Sci. Tech. 2012;7:1-15. [24] Larfeldt, J., Leckner, B., Melaaen, M. C., Modelling and measurements of the pyrolysis of large wood particles. Fuel 2000;79:1637-1643. [25] Blasi, C. D., Physico-chemical processes occurring inside a degrading two-dimensional anisotropic porous medium. Int. J. Heat Mass Transfer 1998;41:4139-4150.

Table 1

Ultimate analysis of raw wood of ramin and Japanese cypress (dry base) [wt%]


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Ramin C H O N S

Table 2

43.9 5.6 50.2 0.3 -

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Japanese cypress 51.5 6.2 42.2 0.1 -

Specific surface area of chars determined by CO2 adsorption at 273 K

X [-] CO2-DR [m2/g] CO2-DFT [m2/g]

Japanese cypress char 0 0.6 641 769 527

834

Ramin char 0 549

0.6 759

498

936


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Reaction tube

To tar trap

Valve

Biomass sample

Ar Infrared furnace

Flow meter

Valve

O2

Valve Vacuum pump Fig. 1

Thermobalance

PC

Schematic diagram of thermobalance


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φ 5 mm

φ 5 mm

500 µm

500 µm

(a) Japanese cypress

(b) Ramin

Fig. 2

2D X-ray CT images of raw wood


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1 mm

y

x z

(a) Japanese cypress char 1mm

y

x z

Fig. 3

3D X-ray CT images of initial char at X = 0 before gasification

(b) Ramin char


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200 µm

200 µm

(1) x-y (pores along z direction)

(1) x-y (pores along z direction)

23 µm

(2) y-z (pores along x direction)

(2) y-z (pores along x direction)

38 µm

(3) x-z (pores along y direction)

(3) x-z (pores along y direction)

(a) Japanese cypress char Fig. 4

(b) Ramin char

2D X-ray CT images of center of initial chars (X=0)


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Fig. 5

Raman spectra of chars before gasification (X=0)


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Fig. 6

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Arrhenius plot of char reactivities


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(a) Japanese cypress

3 mm

Frames of initial chars

Grain direction

Side

Side

Side Bottom

Transverse direction Top Raw wood

Top X = 0 (after pyrolysis)

Top X = 0.6 (gasified at 1173 K)

(b) Ramin

3 mm

Frames of initial chars

Grain direction

Side

Side

Side Bottom

Transverse direction

Top Raw wood

Fig. 7

Top X = 0 (after pyrolysis)

Top X = 0.6 (gasified at 1173 K)

Photographs of raw wood, initial chars (X=0), and chars at X = 0.6 gasified at 1173 K (left to right)


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(b) 2D

200 µm

(1) x-y

(a) 3D

(2) y-z

(3) x-z

Fig. 8

X-ray CT images of center of ramin char gasified at 1173 K (X=0.6)


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(a) X=0

(b) X=0.6

Fig. 9

Measured incremental surface area of chars at X = 0 and X = 0.6 gasified at 1173 K


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