Catalytic Effect of Biomass Pyrolyzed Char on the Atmospheric

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Catalytic effect of biomass pyrolyzed-char on the atmospheric pressure hydrogasification of Giant Leucaena (Leucaena leucocephala) wood Kodchanipa Maneewan, Supachita Krerkkaiwan, Sasithorn Sunphorka, Tharapong Vitidsant, and Prapan Kuchonthara Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie501805e • Publication Date (Web): 09 Jul 2014 Downloaded from http://pubs.acs.org on July 14, 2014

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Industrial & Engineering Chemistry Research

Catalytic effect of biomass pyrolyzed-char on the atmospheric pressure hydrogasification of Giant Leucaena (Leucaena leucocephala) wood

Kodchanipa Maneewan†, Supachita Krerkkaiwan†, Sasithorn Sunphorka†, Tharapong Vitidsant†,‡ and Prapan Kuchonthara*,†,‡

†Fuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Bangkok 10330, Thailand ‡Center of Excellence on Petrochemical and Materials Technology, 8th floor, Petroleum and Petrochemical College Building, Chulalongkorn University, Phayathai Road, Patumwan, Bangkok 10330, Thailand *

Corresponding author. Tel.:+66 2218 7519; fax: +66 2255 5831; E-mail: [email protected]

1 ACS Paragon Plus Environment


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Abstract: This work explored the activity of biomass pyrolyzed-char on the pyrolysis and hydrogasification of Giant Leucaena (Leucaena leucocephala) wood. The effect of char preparation condition, as a slow or fast heating pyrolysis at three different temperatures (650, 750 or 850 oC), on carbon conversion and gas product distribution was investigated. The presence of external hydrogen (H2) had a minor effect on carbon conversion but significantly influenced the gas product distribution. The addition of H2 without pyrolyzed-char favored carbon monoxide hydrogenation followed by water gas shift reaction and resulted in higher levels of methane (CH4) and carbon dioxide (CO2). In the presence of pyrolyzed-char, the addition of H2 had a different effect on gas composition with an increased CH4 via the carbonhydrogen reaction. Moreover, the slow heating pyrolysis at moderate temperature (~ 750 OC) during char preparation provided the most reactive char resulting in tar reduction and increase of CH4 production.

Keywords: Hydrogasification; Pyrolysis; Giant Leucaena; Char; Methanation; Biomass


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1. INTRODUCTION The finding of alternative fuel resources, especially sustainably renewable ones, is a current global challenge. The fuel gas produced from coal and especially from renewable and environmentally sustainable biomass is one such potential alternative fuel that can be produced through several thermal conversion processes. Gasification is one such way to convert solid fuel, including coal and biomass, into a gaseous fuel

1, 2

and is a promising technology for high

energy conversion efficiency 3, 4. Hydrogasification has been developed in order to increase the gas production level, especially methane (CH4), by introducing external hydrogen (H2)

5-10

into the gasification

process. In addition to the increased net gas production, rather than tar, the enhanced CH4 production level increases the heating value of the fuel gas. The gaseous fuel which is produced by this technique is considered as a substitute for natural gas (SNG) 6, 7, 11. Currently, most of the research into hydrogasification of both coal and biomass has been performed under a high H2 pressure

5-7, 9, 12, 13

, where coal hydrogasification is facilitated by the increased H2 pressure

yielding a higher CH4 production rate

6, 12

. However, the required severe operating condition,

including a high temperature and H2 pressure, causes a high level of energy consumption and several safety concerns. Therefore, hydrogasification at a mild condition is more attractive, but it requires the addition of a suitable catalyst in order to increase the otherwise unacceptably low reaction rate. Previously, a Ni-based catalyst with the promotion of calcium

13, 14

and the binary

and ternary eutectic catalytic salts of Na-K-Li 15 have been used in the hydrogasification of wood char and coal at a mild temperature and pressure. Both catalysts increased the overall gasification rate effectively. Sheth et al. reported that the ternary eutectic salts of Na-K-Li and binary of Na-



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K catalysts gave the increased overall gasification rate of coal up to the carbon conversion of 80 – 90%. However, at higher H2 pressure, the catalytic performance was diminished by the inhibition of H2 15. Nevertheless, the metal catalyst and catalyst solution have the disadvantages of their deactivation, especially metal sintering, separation and relatively expensive costs. Recently, char derived from the pyrolysis of coal or biomass, called “pyrolyzed-char”, was used as the catalyst in the gasification process in order to eliminate the tar product. Its attractiveness is its cheap price and that it can be produced simultaneously with the tar reduction inside the gasifier by controlling the reaction parameters and configurations

16

. Moreover, char

has been reported to be able to act as a good catalyst or catalyst support for tar cracking

17-21

.

Zhang et al. 19 studied the rapid pyrolysis of brown coal with the co-feeding of char and reported that the tar yield decreased with increasing char concentrations, while char-supported iron catalysts showed a high activity for the tar reforming reaction during mallee wood gasification 21. The operating conditions during char preparation, such as the heating rate and pyrolysis temperature, were reported to be the important factors determining the subsequent surface structure and char reactivity

22, 23

, where the different char structures and reactivity directly

influenced the catalytic performance of the char for tar decomposition. However, the use of pyrolyzed-char as a catalyst for the hydrogasification process has not been evaluated yet. Therefore, this work explored the effect of biomass pyrolyzed-char on the atmospheric pressure hydrogasification of Giant Leucaena (Leucaena leucocephla) wood in terms of the product distribution and gas composition from hydrogasification in a two-stage fixed bed reactor. Moreover, the effect of the char preparation condition, as a slow or fast heating and the pyrolysis temperature, was examined. The char surface structure was characterized by


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Brunauer–Emmett–Teller (BET) and scanning electron microscopy (SEM) analyses, and these results are discussed together with the catalytic behavior of the char during the atmospheric hydrogasification process.

2. MATERIALS AND METHODS 2.1 Biomass characterization and preparation. Giant Leucaena wood from PTT Research and Development Center, Thailand was used as the feedstock. The proximate and ultimate analyses were performed following ASTM D3172-3175 and using a CHN analyzer (LECO CHN-2000), respectively, and are summarized in Table 1. The Giant Leucaena wood sample was ground, sieved to a 250–500 µm size and then dried at 110 oC for at least 6 h and kept in a desicator before use. 2.2. Char preparation. Char was prepared from Giant Leucaena wood by pyrolysis at a slow or fast pyrolysis rate. The specific surface area, pore volume and pore size of the pyrolyzed chars were measured by N2 adsorption at -196OC using the Brunauer-Emmitt-Teller, BET method (model Quantachrome, Autosorb-1, instrument accuracy ±0.11%) by degassing of sample before adsorption at 300 OC for 6 h. The morphology of the chars was also characterized by scanning electron microscopy (SEM, model JEOL, JSM-5410LV) method. The slow pyrolyzed-char was prepared using a typical fixed-bed reactor as reported previously elsewhere 24. The reactor vessel (20 mm ID) was made from quartz glass. Dry Giant Leucaena wood (3.5 g; equivalent to a 5 cm bed height), was loaded into the reactor and argon (Ar) gas was passed through the bed at a flow rate of 100 mL/min for 2 h. After that the temperature was increased up to the desired temperature (650, 750 or 850 oC) with a heating rate



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about 27 OC min-1 and held at the desired temperature for 30 min allowing the biomass to initially be slowly pyrolyzed as the reactor heated up. These slow pyrolyzed-chars that were prepared at 650, 750 and 850 OC were designated as SC650, SC750 and SC850, respectively. The fast pyrolyzed-char was prepared in the same reactor and conditions as the slow pyrolyzed-char except that the reactor was preheated to 750 oC and left to reach equilibrium with Ar gas at a flow rate of 100 mL/min for 30 min prior to feeding the Giant Leucaena wood in. Thus, fast pyrolysis took place essentially immediately after the wood had been fed in from the top of the reactor. The fast pyrolyzed-chars prepared at 750 OC were designated as FC750. 2.3. Hydrogasification of Giant Leucaena wood. The Giant Leucaena wood gasification was performed in a two-stage fixed bed reactor (Fig. 1). The quartz reactor consisted of an inner tube (8.8 mm ID) and an outer tube (11.8 mm ID) that was separated into two zones, each with individual furnaces and temperature controllers. The upper zone was defined as the pyrolysis zone while the lower zone was the cracking zone with or without external H2. Inactive alumina balls (7.5 g) and 0.5 g pyrolyzed char were packed into the lower zone giving an approximately 2 cm bed height. The Ar and H2 mixture (8.33% v/v H2 balance in Ar) was fed into the reactor at a constant total flow rate of 120 mL/min. The temperature of both zones was adjusted to 700 oC. After the temperature reached the desired values, 0.5 g fresh Giant Leucaena wood was dropped into the inner tube (the pyrolysis zone) and the obtained solid char remained on the quartz wool at the middle of the inner tube whilst only the volatiles from this zone passed into the lower zone. At the lower zone, hydrogasification could take place by the reaction between the biomass derived volatile and external H2 with/without the presence of pyrolyzed-char. The


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gaseous product was then passed through the iced-tar trap. The non-condensable gases, including CH4, carbon monoxide (CO), CO2 and H2, were collected using a gas bag and were further analyzed by gas chromatography with a thermal conductivity detector (GC-TCD) using a Unibeads C packed column. The injection temperature and column temperature were set at 120 o

C and 50–180 oC, respectively. Furthermore, the carbon conversion into gas was determined

following Eq. (1), Carbon conversion into gas (%) =(

, ,

) × 100

(1),

where , represents the mole of carbon in gas product generating from biomass and , represents the mole of carbon in biomass feed.

3. RESULTS AND DISCUSSION 3.1. Effect of the pyrolyzed-char on the pyrolysis and hydrogasification of Giant Leucaena wood. Carbon conversion during the pyrolysis (no externally added H2) and hydrogasification (with externally added H2) of Giant Leucaena wood with and without the SC750 char are summarized in Fig. 2. Note that the reported data are the net results after removal of the products released from the pyrolyzed-char. The level of carbon conversion into gas and tar during hydrogasification was comparable to that in pyrolysis, which indicates that the addition of H2 had only a minor effect on the carbon conversion level. In contrast, the presence of the SC750 pyrolyzed-char had a significant effect on the carbon conversion level in both the pyrolysis and hydrogasification cases, where the level of carbon conversion into gas was about 18% to 25% enhanced, whilst the carbon conversion into tar was ~ 21% reduced. This emphasizes the catalytic effect of pyrolyzed-char on tar reduction, as previously reported 16, 21, 23.



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The gas product distribution from the pyrolysis and hydrogasification of Giant Leucaena wood with and without the SC750 pyrolyzed-char is summarized in Fig. 3. The addition of external H2 resulted in an increased level of CH4 (5.8% higher in cases of with or without SC750 char) and CO2 (9.0% higher in cases of without and with SC750 char, respectively) and about 8.8% decreased level of CO production. This suggested that the hydrogenation of CO and watergas shift (WGS) reactions primarily took place during hydrogasification, as expressed in Eqs. (2) and (3): CO hydrogenation reaction:

CO + 3H 2 → CH 4 + H 2 O

(2)

WGS reaction:

CO + H 2 O → H 2 + CO 2

(3)

However, the effect of the addition of H2 on the gas product distribution was different between that with and without the pyrolyzed-char, where H2 addition increased the CH4 yield by 10% without char and 16% with the pyrolyzed-char. This enhanced CH4 production level could not be explained by Eqs. (2) and (3) because the reduction in the level of CO did not correspond to the increased level of CH4 production. This then implies that the catalytic role of the pyrolyzed-char was involved in the hydrogenation of carbon sources. From previous studies 15, 17, 19, 23-26

, the catalytic gasification of biomass in the presence of char was proposed to occur via

two mechanistic steps. Firstly, is the tar deposition to form coke and then, secondly, the coke was gasified into gas products in both the pyrolysis and gasification conditions. Therefore, the formed coke by tar deposition could attack with hydrogen in hydrogasification via carbon hydrogenation to generate CH4, as expressed in Eq. (4). Carbon hydrogenation:

C + 2H 2 → CH 4

(4)


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3.2. Effect of the wood pyrolysis condition (slow or fast pyrolysis) during the char preparation. The effect of slow and fast pyrolyzed-char at 750 oC on the carbon conversion from the hydrogasification of Giant Leucaena wood is summarized in Fig. 4, where both the slow (SC750) and the fast (FC750) pyrolyzed-char gave a good catalytic performance for tar reduction with an increased carbon conversion level into gas and a decreased level of carbon conversion into tar. However, the SC750 had an apparently better catalytic activity for tar reduction than the FC750 char (23% and 19% reduction for the SC750 and FC750 char, respectively), which can be explained by the differences in the char surface structure, as revealed by the SEM and BET analyses. Representative SEM images of the SC750 and FC750 chars obtained before and after hydrogasification at 700 OC are shown in Fig. 5. The surface structure of FC750 was clearly changed after hydrogasification, with the pore structure evidently collapsed resulting in the loss of porosity, as supported by the 94% and 90% reduction in the BET surface area and pore volume, respectively (Table 2). It has previously been found that fast pyrolyzed-char had a predominant tar deposition to form coke leading to the less char reactivity22. The rupture of the pore structure in FC750 during hydrogasification might be caused by coke formation, resulting in the shallow pores. Presumably, when the coke formed on FC750 it then had less active sites for the secondary reactions to generate gaseous products, as evidenced by the high carbon conversion into tar. On the other hand, the surface structure of SC750, which had a 66% and 39% lower BET surface area and pore volume, respectively, than FC750, did not change as much after hydrogasification with reduction in the BET surface area and pore volume about 94% and 88%, respectively, and large channels of solid cells still being evident. The relative stability of



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the SC750 structure, therefore, significantly influenced its catalytic activity for tar reduction and gas production. The gas product distribution from the hydrogasification of Giant Leucaena wood with the slow (SC750) or the fast (FC750) pyrolyzed-char or with no char is summarized in Fig. 6. Although both chars increased the CH4, CO and CO2 production levels, the SC750 char provided a higher CH4, CO and CO2 production level than did the FC750 char. Presumably the hydrogenation of coke on the FC750 char was inferior to that on the SC750 char due to the lower activity of the formed coke.

3.3. Effect of the pyrolysis temperature during char preparation. The effect of the pyrolysis temperature during char preparation on the subsequent carbon conversion level during the hydrogasification of Giant Leucaena wood is summarized in Fig. 7. It was found that the presence of SC750 provided the lowest level of carbon conversion into tar and corresponded to the highest level of carbon conversion into gas. The SC650 and SC850 chars gave essentially the same level of carbon conversion into gas or tar as each other, but less than that of the SC750 char. Accordingly, the SC750 char provided the best catalytic activity for tar reduction in the hydrogasification process. This could reflect the difference in the char surface structure, as supported by the SEM images of the different chars (Fig. 8), where the surface structure of SC650 after hydrogasification was evidently ruptured and changed into a smoother surface with a loss of active sites for tar decomposition

19, 27, 28

. This was also supported by the lower BET

surface area and pore volume of the spent SC650 char than the spent SC750 char (Table 2), with a 75% and 43% decreased BET surface area and pore volume, respectively, of the spent SC650


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compared to the unused SC650 char. The transformation of the surface structure of SC750 and SC850 after hydrogasification looked broadly similar with shallow pores and a little distortion of the solid cell, and they also had a similar BET surface area and pore volume. Even though this phenomenon could not explain the different catalytic activity of the SC750 and SC850 chars for tar reduction (Fig. 7), the loss of alkali and alkaline earth metallic (AAEM) species on the char surface could be another potential reason. From the result of char characterization by X-ray diffraction (XRF, BRUKER model S8Tiger), revealed that the K and Mg contents on the SC850 were significantly lower than that the SC750 ( 12% and 79% reduction of K and Mg contents, respectively) due to the high thermal treating during the char preparation step. This result was supported by the essentially loss of AAEM on the surface of pyrolyzed-char when preparing the char at high pyrolysis temperature ( > 800 OC) in the previous studies 23, 29, 30. They also reported that the AAEM on char were the catalytic species for tar reduction and char gasification. In addition, the presence of volatiles and excess H2 in the hydrogasification can cause a loss of the AAEM on the char surface 19, 23, 31, making the loss of AAEM on the SC850 char surface a likely major reason causing the lower catalytic activity on tar reduction. The gas product distribution obtained from the hydrogasification of Giant Leucaena wood in the presence of the different chars is shown in Fig. 9. All the different slow pyrolyzedchars gave a higher CH4, CO and CO2 production level than without any chars, but the highest yields of CH4 and CO were obtained from the hydrogasification with the presence of SC750, whilst essentially the same level of CO2 was produced from all three slow pyrolyzed-chars. This highlights that the SC750 char can catalyze the tar decomposition through the CO hydrogenation reaction together with coke hydrogenation, as described in the previous section.



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4. CONCLUSION The pyrolysis and hydrogasification of Giant Leucaena wood was investigated in the presence of various biomass pyrolyzed-char forms at atmospheric pressure and compared to that without any char. The addition of external H2 to promote the hydrogasification had no effect on the carbon conversion level but had an effect on the gaseous product distribution through promoting CO hydrogenation and so resulting in an increased CH4 and CO2 production level. Biomass pyrolyzed-char favored tar cracking, which increased the gas yield, and likely accelerated the methanation reaction during hydrogasification. The heating treatment (slow or fast pyrolysis) and pyrolysis temperature during the char preparation were found to significantly influence the resultant catalytic activity of the char in both the pyrolysis and hydrogasification conditions. Char prepared by slow pyrolysis at 750 oC (SC750) provided a higher gas production level compared to that prepared by fast pyrolysis at 750 oC (FC750) and a higher catalytic performance in the hydrogasification reaction. In addition, the char prepared at 750 oC with slow pyrolysis (SC750) was the most effective for hydrogasification.

ACKNOWLEDGEMENT The authors appreciate the financial support from the Postdoctoral Fellowship (Ratchadaphiseksomphot Endowment Fund), Graduate School, Chulalongkorn University and the Fuel Research Center, Department of Chemical Technology, Chulalongkorn University. The


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authors also wish to express their thanks to Dr. Robert Douglas John Butcher (Publication Counseling Unit, Faculty of Science, Chulalongkorn University) for English language editing.



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Table (s) caption Table 1 Proximate and ultimate analyses of the Giant Leucaena wood Table 2 BET surface analysis of the biomass pyrolyzed-char obtained from the slow (SC) or fast (FC) pyrolysis at 650, 750 or 850 oC

Figure(s) caption Figure 1. Schematic diagram of the two-stage fixed bed reactor that consisted of (1) H2 mass flow controller, (2) and (3) Ar mass flow controller, (4) electric furnaces, (5) quartz reactor, (6) sample feeder, (7) iced-tar trap, (8) bubble flow meter and (9) moisture trap (filled with silica gel).

Figure 2. Carbon conversion level (as % (w/w)) from the pyrolysis and hydrogasifcation of Giant Leucaena wood with and without (w/o) the addition of the SC750 pyrolyzed-char.

Figure 3. Gas product distribution from the pyrolysis and hydrogasification of Giant Leucaena wood with and without the SC750 pyrolyzed-char.

Figure 4. Carbon conversion level (as % (w/w)) in the hydrogasification of Giant Leucaena wood without and with the slow (SC750) and fast (FC750) pyrolyzed-char.


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Figure 5. Representative SEM images (2000 x magnification) of the (a, c) slow pyrolyzed-char (SC750) and (b, d) fast pyrolyzed-char (FC750) obtained (a, b) before hydrogasification and (c, d) after hydrogasification.

Figure 6. Gas product distribution from the hydrogasification of Giant Leucaena wood without and with the slow (SC750) and fast (FC750) pyrolyzed-char.

Figure 7. Carbon conversion level (as % (w/w)) in the hydrogasification of Giant Leucaena wood without and with the slow pyrolyzed-chars (SC) obtained from different pyrolysis temperatures.

Figure 8. Representative SEM images (2000 x magnification) of the (a, d) SC650, (b, e) SC750 and (c, f) SC850 slow pyrolyzed-chars obtained (a–c) before hydrogasification and (d–f) after hydrogasification at 700 oC.

Figure 9. Gas product distribution from the hydrogasification of Giant Leucaena wood without and with the slow pyrolyzed-chars obtained from different pyrolysis temperatures.



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Table 1 wt.% Proximate analysis Moisture

9.69

Volatile

79.5

Ash

2.06

Fixed carbon

8.78 Ultimate analysis*

C

50.0

H

6.19

N

0.80

S**

0.14

O (by difference)

42.9

* daf = dry ash free ** by bomb washing method (ASTM 3177)

Table 2 Char

BET surface area

Pore volume

Pore size

(m2/g-char)

(cm3/g-char)

(Ao)

SC650

1.88

0.0073

146.8

SC750

2.18

0.0092

131.4

SC850

2.07

0.0089

143.2

FC750

5.16

0.0166

128.8

Unused

Spent (after the reaction) SC650

0.47

0.0042

384.2

SC750

0.73

0.0056

307.7

SC850

0.64

0.0051

357.1

FC750

0.32

0.0020

484.0

ACS Paragon Plus Environment

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Figure 1.

Figure 2.



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Figure 3.

Figure 4.


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(a)

(b)

(c)

(d)

Figure 5.



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Figure 6.

Figure 7.


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(a)

(b)

(c)

(d)

(e)

(f)

Figure 8.



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Figure 9.


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Catalytic Effect of Biomass Pyrolyzed Char on the Atmospheric

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