Simultaneous Steam Reforming of Tar and Steam Gasification of Char

ARTICLE pubs.acs.org/EF

Simultaneous Steam Reforming of Tar and Steam Gasification of Char from the Pyrolysis of Potassium-Loaded Woody Biomass Tsukasa Sueyasu, Tomoyuki Oike, Aska Mori, Shinji Kudo, Koyo Norinaga, and Jun-ichiro Hayashi* Institute for Materials Chemistry and Engineering, Kyushu University, 6-1, Kasuga Koen, Kasuga 816-8580, Japan ABSTRACT: This paper proposes a two-stage conversion of biomass into gas, which consists of pyrolysis at 500
600 °C and steam reforming/gasification at 600
700 °C, and has a special feature of recycling of the potassium (K) catalyst. The proposed process was simulated experimentally employing K-loaded cedar as the feedstock and char from its pyrolysis as the catalyst for tar reforming. Tar from the pyrolysis was reformed over the char in a sequence of carbon deposition onto the pore surface and K-catalyzed steam gasification of the deposit, while K-catalyzed char gasification created active pores simultaneously. At the steam/carbon molar ratio of 0.55
1.10, the catalysis of K simultaneously realized the concentration of heavy tar (boiling point temperature > 336 °C) in the product gas as low as 20 mg m3N dry and progress of the char gasification as fast as that of char formation by the pyrolysis. The concentration of hydrogen in the product gas exceeded 50 vol % dry. A portion of K was released from the pyrolyzing cedar, fully captured by the char bed of the reformer, and involved in the steam reforming and gasification. A major portion of K retained in/on the spent char was extracted with water. The resulting aqueous solution of K was ready to be used as spray for K loading on the feedstock.

1. INTRODUCTION

was greater than that of carbon deposition. It was also found that increasing the concentration of hydrogen in the gas phase suppresses the char activity, reducing the rates of carbon deposition and steam gasification. Kimura et al.10 examined air/steam reforming of tar from the pyrolysis of woody biomass in a fixed bed of char that had been prepared by pyrolyzing the same parent biomass. They reported that a temperature as high as 850 °C was needed for maintaining the char activity high enough to reduce the residual yield of heavy tar (heavier than phenanthrene with a boiling point temperature of 336 °C) to 0.01
0.02 wt % of the feedstock biomass. Steam gasification of the char occurred simultaneously with the tar reforming at a rate higher than that of carbon deposition from the tar. The degree of the net gasification of char was, however, limited within 20% of the char formed by the pyrolysis. Brandt et al.6 investigated two-stage conversion, inserting partial oxidation of the volatiles with air at 1050
1100 °C between the pyrolysis of biomass and reforming in a fixed bed of char. The temperature of the top part of the char bed was estimated to be 950
1000 °C. The combination of the gas-phase partial oxidation and the subsequent reforming over the char enabled complete elimination of heavy tar and also reduction of the naphthalene yield to even below 0.001 wt %. 1.2. Application of the Potassium Catalyst to Tar Reforming over Char. As mentioned above, the two-stage conversion is an effective and practical way for gasifying biomass minimizing tar emission, but it seems to need a reforming temperature even as high as 1000 °C. In addition, it would be a difficult task to

1.1. Decomposition of Tar over Char in Biomass Gasification. Removal of tar has been one of the most important

technical subjects in the development of biomass gasification for syngas production and power generation.1,2 Efforts have been made to eliminate tar from the product gas, in particular, inside the gasifier.3,4 Char is a major product from the pyrolysis that is the primary step of gasification, in other words, an important intermediate of the gasification. It is known that contact between vaporous tar and char is effective to decompose tar at temperatures higher than 800 °C.3,5
10 Results of previous studies7,9 suggest that carbon deposition onto char was responsible for the fast decomposition of tar. There have been a number of proposals of biomass gasification with natural catalysts, such as dolomite and olivine, and synthesized catalysts, such as Ni
Al2O3, for tar removal. Application of these catalysts has not necessarily been successful because of insufficient reduction of the tar concentration11 or catalyst deactivation and poisoning for the natural and synthesized catalysts, respectively.12,13 A definite advantage of using char as an active tar decomposition promoter is that the char is produced from the pyrolysis of feedstock. In addition, deactivation of char, even if it has occurred, is not a serious problem because spent char can be used as a solid fuel or material. However, there remain subjects toward the improved performance of char. Hosokai et al.9 investigated decomposition of mono- and polyaromatic compounds over char from woody biomass. The initial activity of the char was extremely high that more than 99.9% of benzene and naphthalene were decomposed at 800
900 °C, but such high activity diminished because of consumption of micropores that served as active sites for the carbon deposition from the aromatics. Progress of steam gasification simultaneously with the carbon deposition helped the char maintain its activity, forming and/or regenerating micropores if the rate of gasification r 2011 American Chemical Society

Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 1, 2011 Revised: October 6, 2011 Published: October 07, 2011 199

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Figure 1. Conceptual diagram of the two-stage conversion of biomass into syngas with recycling of the potassium catalyst.

achieve complete char gasification simultaneously with complete tar reforming unless a high temperature and/or high air/oxygen ratio are allowed.9,10,14 This paper proposes a two-stage conversion of biomass with the use of a potassium (K) catalyst for steam reforming of tar and steam gasification of char at a temperature of 700 °C or lower and recycling of the catalyst. The primary purpose of employing the K catalyst is to cause steam gasification of char that is indispensable to maintain the char activity toward carbon deposition from tar vapor as well as complete gasification of the char at such a low temperature as above. There have been reports on the catalytic effects/roles of K in steam gasification of biomass char since Mudge et al.15 reported steam gasification of char from the pyrolysis of K2CO3-loaded woody biomass. However, little is known about K-catalyzed gasification of char surrounded by volatiles from the pyrolysis and steam reforming of the volatiles over gasifying K-loaded char. Figure 1 illustrates a conceptual diagram of the proposed process. A catalyst precursor is loaded on the feedstock biomass by spraying an aqueous solution of potassium salt, such as K2CO3. The K-loaded biomass is pyrolyzed into char (hereafter referred to as K-char) and volatiles (noncondensable gas, steam, and tar). The pyrolysates are introduced into the reformer together. The reformer consists of a moving or fixed bed of K-char that is continuously supplied from the pyrolyzer. The tar is steamreformed within the K-char bed, while the char is steam-gasified under catalysis of K. The K-char is discharged from the reformer bottom at a certain rate, so that the bed volume is maintained steady and large enough for the steam reforming and gasification. The discharged K-char is washed with water for extracting K and recycling it in a form of aqueous solution. The water-washed spent char is recycled to the reformer or otherwise burned to supply heat to the reformer/pyrolyzer or forwarded as a product. It is known that alkali metallic species in/on the char is more or less volatile during steam gasification of char,16
20 undergoing repeated desorption and adsorption from/onto the char surface but remaining inside the reformer. It is thus expected that the recycled char is in situ loaded with K. No or little carryover of K to the

reformer downstream is preferred for easier product gas treatment. It is also known that K is released into the gas phase during the pyrolysis of biomass.19
23 Volatilized K species are transferred into the reformer and then adsorbed onto the recycled char. K species catalyze not only steam gasification of char but also pyrolysis of the feedstock biomass, causing a decrease in the yield of tar to be reformed while an increase in that of char to be gasified.24
26 In the case of oxygen or air feeding into the reformer with or without steam, partial combustion of the volatiles will take place above the K-char bed, forming high-temperature gas, and it will be quenched not physically but chemically by the steam reforming of the residual tar and steam gasification of the K-char. Fast progress of such endothermic reactions will cause a temperature gradient along with the bed axis, allowing the product gas to exit the reformer at a temperature as low as 700 °C or even lower. On the other hand, external heating of the reformer may be possible if the reformer is slender enough to allow for fast heat transfer via the reactor wall. In such a case, steam is fed into the reformer, while hot gas from combustion of a portion of the produced gas or another fuel (e.g., char or feedstock biomass) flows around the reformer, giving it heat. 1.3. Purpose of This Work. This work primarily aimed to examine the potentiality of simultaneous complete steam reforming of tar over K-loaded char and its steam gasification at temperatures lower than 700 °C. The pyrolysis of K-loaded woody biomass, steam reforming of the volatiles, and steam gasification of the K-loaded char were simulated experimentally with a two-stage reactor system at different steam/biomass ratios. The secondary purpose was to investigate behaviors of K species in a sequence of K loading on the biomass, the pyrolysis of K-loaded biomass, steam reforming/gasification of volatiles/ K-char, and extraction with water of K from spent K-char.

2. EXPERIMENTAL SECTION 2.1. Material. Chipped Japanese cedar was used as the biomass feedstock. Its carbon, hydrogen, and oxygen contents were 50.9, 6.4, and 200

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Figure 2. (a) Apparatus for two-stage conversion of biomass. (b) Explanation of the material fed to or charge into reactors and collected products. 43.2 wt % on a dry basis, respectively. The average size of chips was 10  10  2 mm. The feedstock was dried in air at 110 °C for 24 h prior to use. Aqueous solutions of K2CO3 with different concentrations were prepared from K2CO3 of a reagent-grade and purified water (electrical resistance = 18.2 MΩ). The loading of K was performed by spraying a K2CO3 solution on 200 g of dry cedar chips, which were agitated in a vessel (diameter, 320 mm; depth, 190 mm). The amount of the K2CO3 solution was determined, so that the resultant K-loaded cedar had a K content of 1.49 wt % K of the dry cedar, while the water content varied with the K2CO3 concentration of the solution employed. Some K-loaded cedar samples were further subjected to drying for reducing the water content below 1 wt %. The water content of the K-loaded cedar samples ranged from 0.86 to 45.7 wt % on a wet sample basis. The K-loaded cedar is hereafter referred to as K-cedar. Char samples were prepared by pyrolyzing K-cedar or the original cedar. Details of the char preparation will be explained later.

dropped into the collector vessel at an ambient temperature, and the volatile products and steam from the moisture of K-cedar were introduced into the reformer. N2 was continuously supplied from the char collector toward the reformer (via the pyrolyzer) at a flow rate of 0.916 LN min
1 for avoiding diffusion of volatiles into the char collector. N2 was also supplied to the reformer from its bottom at a rate of 0.275 LN min
1 because of a reason similar to that above. As mentioned above, K-char formed in the pyrolyzer was not fed directly into the reformer. Instead, the reformer (SUS304, inner diameter = 55 mm) was charged with 100 g of K-char prior to the experiment to form a fixed bed with a height of about 290 mm. The K-char was prepared separately by the pyrolysis of K-cedar but in the same way as mentioned above. Thus, the reforming was performed with a fixed bed of K-char instead of a moving bed. As illustrated in Figure 3, the temperature of the K-char bed distributed over a range from 600 °C (top and bottom) to 700 °C (100 mm below the top). There were two different sources of steam. One was the moisture of K-cedar, and the other was pyrolytic steam (steam formed by the pyrolysis). Neither O2 nor air was used for the reforming. The gas residence time depended upon the moisture content of K-cedar, in other words, feeding rate of the steam source, ranging from 1.7 to 3.3 s on an empty bed basis. Some runs were performed without the reformer for investigating the distribution of the products from the pyrolysis (i.e., before the reforming) and for preparing K-char to be used as the bed material. Pyrolysis of the original cedar was carried out, and the product yields were compared to those from the pyrolysis of K-cedar for examining effects of K-loading on the pyrolysis characteristics. 2.3. Product Recovery and Analyses. The volatile products from the reforming, i.e., noncondensable gas, tar, and steam, were

2.2. Experimental Simulation of Two-Stage Conversion of Biomass. Figure 2a shows a schematic diagram of the apparatus that was used for simulating the two-stage conversion of K-cedar. The apparatus had two reactors, a screw-conveyer pyrolyzer and a fixedbed reformer, that were heated independently in electrical furnaces. Prior to heating of these reactors to prescribed temperatures, K-cedar and char from the pyrolysis of K-cedar (K-char) were charged into the biomass hopper and reformer, respectively. The K-cedar was fed into the pyrolyzer at a constant rate of 2.2 g of dry cedar min
1 together with N2 gas (purity > 99.9995 vol %), flowing at 0.458 LN min
1, and heated to 550 °C at a heating rate of about 6 °C s
110 while conveyed through the reactor tube (SUS304, inner diameter = 50 mm). The feeding period was 60 min, unless otherwise noted. The char formed by the pyrolysis 201

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Table 1. Nomenclatures and Subscripts nomenclature Minput

total input of material into the pyrolyzer and reformer

Moutput

total output of material from the pyrolyzer and reformer

mi

amount of product i on a mass, carbon, hydrogen,

m0 i

or oxygen basis corrected amount of product i

subscript K-cedar

K-cedar (moisture is excluded) fed into the pyrolyzer

K-char-R1

K-loaded char (K-char) charged into the reformer

K-char-R2

spent K-char recovered from the reformer

gas-1

gas formed from K-char-R1 during heating of the

gas-2

gas formed from pyrolysis and reforming during feeding of K-cedar

K-char-P

K-char recovered from the char collector vessel

H-tar

heavy tar formed from pyrolysis and reforming

L-tar

light tar formed from pyrolysis and reforming

volatiles water-0

gas-1, gas-2, H-tar, and L-tar water fed into the pyrolyzer as moisture of K-cedar

water-1

water formed from K-char in the course of

water-2

water collected in condensers during feeding of K-cedar

water-P

water produced by the pyrolysis of K-cedar

reformer prior to K-cedar feeding

Figure 3. Axial temperature distribution of the reformer. (A) Height of the K-char bed before heating the reformer. introduced into a train of solid/liquid collectors/condensers. An aerosol filter at 170 °C and three condensers (0,
40, and
70 °C) were connected in series. It was preliminary confirmed that even benzene vapor was condensed mainly in the second and third condensers with a total recovery of 98% or more. The third condenser was packed with glass beads, which were necessary for complete recovery of light liquids. The aerosol filter was indispensable for capturing aerosol particles consisting of a heavy portion of tar, hereafter referred to as heavy tar (H-tar). The lighter portion of the tar, i.e., light tar (L-tar), was condensed in the three condensers. Analyses of the tar by gas chromatography confirmed that L-tar consisted of aromatics ranging from benzene to phenanthrene (boiling point temperature = 336 °C), while H-tar was composed of aromatics with higher boiling point temperatures. Noncondensable gas was collected in gasbags and analyzed by gas chromatography. Inorganic gases (H2, CO, and CO2), light hydrocarbon gases (C1
C4), methanol, and dichloromethane were quantified. The heaviest portion of tar was deposited onto the inner wall of SUS316 tubes between the reformer and aerosol filter. The deposit was fully recovered by washing the tubes with acetone or tetrahydrofuran and defined as a portion of H-tar. The yield of H-tar was determined from its mass and carbon content. GC analyses confirmed that H-tar consisted of pyrene and heavier compounds. L-tar was analyzed by GC, which detected the presence of aromatic compounds ranging from benzene to phenanthrene with respect to the normal boiling point temperature. The char samples were recovered from the char collector and reformer, and their amounts were determined on mass and carbon bases. Some char samples were analyzed by X-ray diffractometry (XRD) on a Rigaku RINT-TTR III diffractometer.

(K-char formed by pyrolysis) during feeding of K-cedar during feeding of K-cedar

heating the reformer prior to K-cedar feeding

was performed in another TGA (SII Nanotechnology, Inc., model EXSTAR TG/DTA6000). The char sample was heated at a rate of 10 °C min
1 up to 710 °C under atmospheric flow of high-purity N2, where the temperature was held. Steam was then fed into the TGA at a flow rate, so that its concentration was maintained at 20 vol %. The main purpose of the analysis was to confirm catalytic activity of K retained in/ on the char. 2.6. Definition of Product and Yield. After recovery and quantification of the products, the material balance on a mass or carbon basis was examined between the total input (Minput) and output (Moutput), which are defined by the following equations:

2.4. Leaching of K from K-Cedar and K-Char Samples with Water and Acid Solution. K-cedar and K-char samples before and after reforming were subjected to leaching of K with an aqueous solution of methane sulfonic acid (CH3SO3H at pH 1.0) or water at 25 °C. A prescribed amount of solid was mixed with the liquid in a plastic bottle, and the mixture was stirred. The K concentration in the aqueous phase became steady within 20 h. The mixture was then filtered, and the filtrate was analyzed by ion chromatography for quantification of K. Details of the K analysis were reported elsewhere.27 The solid residue was dried and subjected to mild combustion in a thermogravimetric analyzer (TGA; Bruker Japan Co., Ltd., model TG-DTA 2000S) under a condition avoiding ignition of the char and resulting in the loss of K from the solid. The combustion was performed according the procedure reported by Sathe et al.28 The resulting ash was dissolved into an aqueous solution of CH3SO3H (pH 1.0) and then subjected to the ion chromatography. 2.5. Steam Gasification of Char. Steam gasification of char samples from the pyrolysis of K-cedar and that from the original cedar

Minput ¼ mK-cedar þ mwater-0 þ mK-char-R1

ð1Þ

Moutput ¼ mK-char-P þ mK-char-R2 þ mgas-1 þ mgas-2 þ mL-tar þ mH-tar þ mwater-1 þ mwater-2

ð2Þ

The nomenclatures and subscripts involved in these equations are explained in Table 1 and Figure 2b. Under the present experimental conditions, Moutput/Minput ratios were reproducible within a range of 0.98
1.02 on both mass and carbon bases. Reproducibility within error of (10% was also confirmed for the yield of H-tar, which was often less than 0.1 wt % of K-cedar fed to the pyrolyzer. As indicated by eq 2, gas was formed not only during the period of K-cedar feeding to the pyrolyzer but also that of heating the reformer up to the reforming temperature. K-char charged into the reformer was prepared by the pyrolysis of K-cedar at 550 °C that was lower than the reforming temperature of 600
700 °C. K-char underwent pyrolysis before exposed to the volatiles and steam from the pyrolyzer. It is supposed in the proposed process that such pyrolysis of K-char takes place because of the same reason as above. It is also postulated that the K-char is introduced directly into the reformer to form a moving bed. On the other hand, 202

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Figure 5. Sum of YK‑char‑P, Ygas‑2, YL‑tar, and YH‑tar as a function of the S/C ratio on a molar basis.

Figure 4. YK‑char‑P as a function of the S/C ratio. The S/C ratio is equal to the ratio of mwater‑0/mK‑cedar.

conversion is defined by under the present experimental conditions, the amount of K-char charged into the reformer, mK‑char‑R1, was more than that formed from the pyrolysis of K-cedar, mK‑char‑P. It was thus needed to correct the amount of gas (gas-1) from the pyrolysis of K-char. The corrected amount of gas-1 is calculated by   mK-char-P ð3Þ m0 gas-1 ¼ mgas-1 mK-char-R1

Xwater, net ¼ 1


3.1. Overall Characteristics of Conversion of K-cedar. Figure 4 plots YK‑char‑P against the steam/carbon (S/C) ratio that is directly given as the mwater‑0/mK‑cedar ratio. YK‑char‑P was almost independent of the S/C ratio. It was concerned that a higher S/C ratio decreased the heating rate of K-cedar because of the requirement of more heat to generate steam from the moisture of K-cedar. In general, a lower heating rate for the biomass pyrolysis results in a higher char yield and lower tar yield.18 It was, however, confirmed that such an effect of the S/C ratio on the char yield, if any, was insignificant under the present experimental conditions. It was then reasonably estimated that the yield and composition of the volatiles to be supplied to the reformer were nearly independent of the S/C ratio. The averaged YK‑char‑P, 47.4% on a K-cedar carbon basis, was higher by 10% than the char yield from the original cedar. It was clear that char formation was promoted by the presence of K species. Although not shown in the figure, K-loading also resulted in suppression of tar formation during the pyrolysis. The yields of H-tar and L-tar from the pyrolysis of the original cedar were 21 and 20% on the cedar carbon basis, respectively, which were decreased by the K-loading to 10.8 and 13.0%, respectively. Figure 5 examines the net progress of steam gasification of K-char in the reformer during the reforming. Net K-char gasification occurs when the overall rate of steam gasification of K-char carbon together with tar-derived carbon deposit is greater than that of the carbon deposition onto K-char. The sum of YK‑char‑P, Ygas‑2, YL‑tar, and YH‑tar was higher than 100% based on K-cedar carbon over the entire range of the S/C ratio and increased monotonously with the S/C ratio. This result proved the progress of net gasification of K-char regardless of the S/C ratio. The sum of YK‑char‑P, Ygas‑2, YL‑tar, and YH‑tar was 107% even at the lowest S/C ratio employed, i.e., 0.01. The formation of pyrolytic steam, the yield of which was as much as 30.2 mol of H2O per 100 mol of C of K-cedar, enabled the progress of such steam gasification of K-char even with little moisture of K-cedar.

The mK‑char‑P/mK‑char‑R1 ratio was 0.40 under the present experimental conditions. The yield of product i, Yi, is defined as follows: mK-char-P YK-char-P ¼ ð5Þ mK-cedar mH-tar mK-cedar

ð6Þ

YL-tar ¼

mL-tar mK-cedar

ð7Þ

Ygas ¼

m0 gas-1 mgas-2 þ ¼ Ygas-1 þ Ygas-2 mK-cedar mK-cedar

ð8Þ

Although not shown here, the yields of the individual gases were defined in the same manner as for eq 8 on either a mass, carbon, hydrogen, or oxygen basis. Formation and consumption of steam is an important parameter for evaluating the overall performance of steam reforming and gasification. The net conversion of steam in the reformer, Xsteam,net, is given by Xwater, net ¼ 1


m0 water-1 þ mwater-2 mwater-0

ð10Þ

3. RESULTS AND DISCUSSION

The pyrolysis of K-char produced not only gas-1 but also water (water-1) prior to the reforming. Then, its amount was corrected in the same way as above.   mK-char-P ð4Þ m0 water-1 ¼ mwater-1 mK-char-R1

YH-tar ¼

m0 water-1 þ mwater-2 mwater-0 þ mwater-P

ð9Þ

Steam from the moisture of K-cedar was not the only source of steam for the reforming and gasification, which were also contributed by steam formed from the pyrolysis of K-cedar. Its amount, mwater‑P, was measured by pyrolyzing the K-cedar without reforming. Then, the gross steam 203

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Figure 6. Conversion of char as a function of time during steam gasification of char samples in TGA. Temperature, 710 °C; steam concentration, 20 vol %; and balance gas, high-purity nitrogen (purity > 99.9999 vol %).

Figure 8. Relationship between the total input of steam and its consumption. The slope of the straight line indicates the overall conversion of steam from the moisture of K-cedar (water-0) and that from its pyrolysis (water-P), i.e., gross steam conversion.

reformer during the reforming, Yvolatiles over 100% on the K-cedar carbon basis, if it occurs, indicates a possibility of 100% steadystate carbon conversion in the proposed process. Figure 7 shows that Yvolatiles was over 105% at S/C ratio = 1.10. It was also estimated that Yvolatiles reached 100% at around 0.9, where the total flow rate of the volatile products was equivalent to the feeding rate of K-cedar on its carbon basis. For the run with S/C ratio = 1.10, 119 mol of C of K-char (K-char-R1) was charged into the reformer per 100 mol of C of K-cedar fed to the pyrolyzer and then 66 mol of C of spent K-char (K-char-R2) was recovered after the reforming. A substantial portion, 45%, of the K-char was gasified during the reforming. More details of this result will be discussed later. Figure 8 plots the amount of steam consumed in the reformer against the total amount of steam from the moisture of K-cedar and pyrolytic steam. Every amount has been normalized by that of K-cedar on its carbon molar basis. The slope of the straight line drawn in the figure indicates the gross conversion of steam in the reformer, Xsteam,gross. This steam conversion was as high as 74% over the entire range of the S/C ratio examined. Such extensive consumption of steam supported the progress of steam gasification of K-char. The steam consumption at the S/C ratio = 1.10 was 141 mol of H2O per 100 mol of C of K-cedar and was much more than the net consumption of K-char carbon, 53 mol of C, in consideration of the following stoichiometry of steam gasification:

Figure 7. Sum of Ygas‑1, Ygas‑2, YL‑tar, and YH‑tar (i.e., Yvolatiles) as a function of the S/C ratio on a molar basis.

The net progress of K-char gasification even in the volatile-rich atmosphere and at temperatures of 600
700 °C arose primarily from catalysis of K. The reactivity of K-char toward steam was investigated at 710 °C by TGA, and the result was compared to that for the gasification of char prepared from the original cedar, as shown in Figure 6. Gasification of K-char was completed within a period shorter than 15 min, leaving inorganic residue. A gradual and apparent increase in the conversion after 15 min was mainly due to volatilization of KOH that had a vapor pressure of about 0.11 kPa at 710 °C. The gasification of char from the original cedar took place much slower than that of the K-char. Thus, it was clear that the progress of net gasification of K-char during the reforming was caused by the catalysis of K species. Figure 7 shows the sum of Ygas‑1, Ygas‑2, YL‑tar, and YH‑tar, hereafter denoted by Yvolatiles, as a function of the S/C ratio. This sum has an important meaning in considering the potential performance of the proposed process. It is expected in the proposed process with continuous operation that the rate of net gasification of K-char in the reformer can be equivalent to that of K-char formation in the pyrolyzer; in other words, the steadystate conversion of K-cedar to volatiles can be 100%, if the amount of K-char in the reformer is maintained at an appropriate level. Although in the present study no K-char was supplied to the

C þ H2 O ¼ CO þ H2 Although not shown in detail, the difference between 141 mol of H2O and 53 mol of C was explained well by steam consumption by steam reforming of L-tar and H-tar and, in addition, that by the water
gas shift reaction. CO þ H2 O ¼ CO2 þ H2 3.2. Steam Reforming of Tar and Formation of Gaseous Products. Figure 9a exhibits changes in YH‑tar with the S/C ratio.

This figure also indicates the H-tar yield from the pyrolysis of K-cedar (without reforming). YH‑tar was about 0.1% at the S/C ratio = 0.01, which was less than 1% of the yield from the pyrolysis alone on the same carbon bases. It was implausible that 204

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Figure 9. (a) YH‑tar as a function of the S/C ratio. (b) Concentration of H-tar in the product gas on a steam/N2-free basis.

Figure 10. (a) YL‑tar as a function of the S/C ratio. (b) Composition of L-tar.

The reforming also reduced YL‑tar but less extensively compared to YH‑tar. This was due to the higher thermochemical stability of L-tar than H-tar.29 It is seen in Figure 10a that YL‑tar changed with the S/C ratio via a maximum. The presence of the maximum yield of L-tar suggested the formation of L-tar by the decomposition of H-tar. It was plausible that increasing the S/C ratio caused an increase in the concentrations of H2 and H radicals in the gas phase and on the K-char surface, respectively, thereby enhancing the product selectivity to L-tar in the decomposition of H-tar. This is a possible explanation of the increase in YL‑tar at a S/C ratio < 0.25. The yield of L-tar decreased as the S/C ratio increased beyond 0.25. Rapid formation of micropores that served active sites for the deposition of L-tar promoted the conversion of L-tar. Another possible explanation is enhancement of direct K-catalyzed steam reforming of L-tar. It was, however, difficult to distinguish such a mechanism, even if important, from a sequence of carbon deposition onto the K-char surface and K-catalyzed gasification of the deposited carbon. Figure 10b shows that L-tar consisted of mono-, di-, and triaromatics ranging from benzene to phenanthrene. A major portion of benzene derivatives was explained by benzene and toluene, while that of the phenols was explained by phenol and cresol isomers. The diaromatics consisted mainly of naphthalene and methylnaphthalene. Acenaphthylene was the most abundant in the triaromatics, while fluorene and phenanthrene were present over the entire range of the S/C ratio. It is noted that the change in the composition of L-tar was insignificant. It was suggested that

such extensive decomposition of H-tar occurred in the gas phase at temperatures lower than 700 °C and, therefore, reasonable to conclude that the surface of K-char was responsible for the H-tar removal. YH‑tar decreased with an increasing S/C ratio to 0.008%, which corresponded to conversion of the initial H-tar (from the pyrolysis) of more than 99.9%. The effect of the S/C ratio on YH‑tar is discussed here. According to previous reports,9,10 it is believed that vapor of H-tar was deposited onto the micropore surface of K-char, forming carbon deposit, light gas, and also L-tar, and the deposited carbon was gasified with steam under catalysis of K species. An increase in the S/C ratio would bring about two different effects on the decomposition of H-tar. First, an increase in the rate of steam gasification of K-char carbon and tar-derived carbon caused a more extensive formation and regeneration of micropores, and this resulted in the promotion of H-tar deposition onto the K-char surface. Second, enhanced steam gasification caused an increase in the concentration of H2 in the gas phase, as well as that of H radicals on the char surface. This resulted in suppression of carbon deposition from H-tar. Increasing the S/C ratio thus induced both positive and negative effects on the decomposition of H-tar, respectively, and the combination of these effects gave such a change in YH‑tar with the S/C ratio, as seen in Figure 9a. The concentration of H-tar in the product gas was calculated from YH‑tar and the total gas yield. The result is given in Figure 9b. The concentration was 21 mg m3N dry at the S/C ratio = 1.10. 205

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Figure 11. Yields of gaseous products as a function of the S/C ratio: (a) inorganic gases and (b) light hydrocarbon gases.

the removal of diaromatics, triaromatics, and/or phenols selectively to benzene and toluene was not easy under the reforming conditions examined and that higher temperatures rather than higher S/C ratios would be necessary for complete removal of L-tar. Although not examined in this work, a reasonable and practical way to eliminate L-tar is to employ internal heating of the reformer, in other words, to feed oxygen or air to the headspace of the reformer. Partial oxidation of the volatiles creates hot gas at temperatures well above 700 °C, and the gas can potentially be quenched chemically to a temperature below 700 °C while traveling through the K-char bed. It is also expected that L-tar is decomposed more extensively than in the present study if the temperature well above 700 °C is provided in the top or upper part of the K-char bed. The yields of gaseous products are shown in panels a and b of Figure 11. The yields of H2, CO, and CO2 increased monotonously with the S/C ratio. The increase in the H2 yield was the most significant on a molar basis. This resulted in an increase in the H2 concentration from 21 to 36 vol % (including steam and N2) as the S/C ratio increased from 0.01 to 1.10. The H2 concentration on a steam/N2-free basis was higher than 50 vol % at a S/C ratio over 0.5. The yields of hydrocarbons changed with the S/C ratio but much less significantly than that of L-tar. Not only CH4 but also C2
C4 hydrocarbons were relatively stable under the range of S/C ratios examined, probably because of weaker propensity toward deposition onto char surface than that of aromatics. The yields of CH4 and C2
C4 hydrocarbons from the reforming were clearly higher than those from the pyrolysis alone. Two reaction pathways may explain this. One is thermal cracking of tar, which occurs even in the gas phase but is also enhanced by the presence of char. The other is hydrogenation of CO, which is possible at the temperature of the reformer in a thermodynamic sense. 3.3. Examination of the Time-Dependent Change in the Product Yields. Experiments were carried out with the S/C ratio = 1.10 and different K-cedar feeding periods of 20, 30, and 60 min. Figure 12 exhibits changes in (m0 gas‑1 + mgas‑2 + mL‑tar + mH‑tar), mH‑tar, and mL‑tar with mK‑cedar. The data for 22, 33, and 67 g of C of K-cedar correspond to those with 20, 30, and 60 min of feeding of K-cedar, respectively. The straight line drawn in Figure 12a indicates Yvolatiles = 100% on the K-cedar carbon basis. Yvolatiles was always over 100% but decreased with mK‑cedar. Yvolatiles values were 153, 150, and 105% at 20, 30, and 60 min, respectively. It was believed that considerable reduction of Yvolatiles at 30
60 min was

Figure 12. Relationships of (a) total of mgas‑1, m0 gas‑2, mL‑tar, and mH‑tar, (b) mL‑tar, and (c) mH‑tar with the amount of K-cedar fed to the pyrolyzer.

mainly due to the loss of K-char carbon because of its steam gasification and resultant decrease in the bed height. The bed height after the 30 min run was 145 mm, which was half of the initial height. According to the temperature profile of Figure 3, it was estimated that no K-char particles were present in the hottest 206

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and AlKO3. These composite oxides are water-soluble or acidsoluble, and therefore, the formation of such composite oxides was not an explanation for the presence of water-/acid-insoluble K. It was rather suggested that a small portion of K was retained in a carbon structure inaccessible to water. Taken together with this fact, the data in Table 2 show a possibility of efficient K recycling in the proposed two-stage conversion of biomass. Incomplete extraction of K with water is not a problem in the proposed process because the spent char can be recycled to the reformer.

Table 2. Balance of K for a Run with S/C Ratio = 1.10 water-soluble water-insoluble classification

notation

milligrams of K

milligrams of K

total milligrams of K

K in K-cedar

A

1930

trace

1930

K in K-char-P

B

1250

100

1350

K volatilized during

A
B

580

pyrolysis (calculated) K in K-char-R1 K in K-char-R2 K in K-char-R2 (calculated)

D E D+

3140 3860

250 250

4. CONCLUSION A two-stage conversion of biomass, which consists of the pyrolysis of K-loaded biomass and simultaneous steam reforming/ gasification of tar/char, was proposed and simulated experimentally, employing a screw-conveyer pyrolyzer at 550 °C and a fixedbed reformer at 600
700 °C, consisting of char from the pyrolysis of the K-loaded biomass. The experimental results revealed simultaneous progress of nearly complete steam reforming of heavy tar over the K-loaded char and its rapid steam gasification. The carbonbased gas production rate was even more than the feeding rate of the K-loaded biomass, showing a possibility of 100% steady-state carbon conversion by the proposed process. It was revealed that the bed of K-char in the reformer captured volatilized K from the pyrolysis and retained it without releasing it downstream. A major portion of K retained by the spent K-char was extracted with water at the ambient temperature. A possibility of efficient recycling of K with minimized loss was thus demonstrated.

3390 4110 3970

(A
B)

zone of the reformer at 700 °C. It also seemed that Yvolatiles was as high as 150% in the early period. Thus, the steam gasification of K-char was sufficiently fast at 700 °C even with the supply of the volatiles from the pyrolysis. Panels b and c of Figure 12 show linear increases in mL‑tar and mH‑tar with mK‑cedar and, therefore, no significant changes in either YH‑tar or YL‑tar. Hosokai et al.9 showed decomposition of aromatics taking place over the char surface forming the carbon deposit, which is then gasified with steam together with the original char carbon. It was also shown that more rapid steam gasification than carbon deposition was necessary for the maintenance of the char activity toward aromatics.9,10 It was confirmed that net gasification of K-char continued for 60 min at the S/C ratio = 1.10; in other words, the steam gasification was faster than the carbon deposition over the entire range of this period. Stable YH‑tar or YL‑tar were thus consistent with the maintenance of the activity of K-char toward H-tar and L-tar, respectively. It was also suggested that such activity was less sensitive to the temperature than the kinetics of steam gasification. 3.4. Chemical Form of the K Catalyst. K-char and spent char, denoted by K-char-R1 and K-char-R2, respectively, were analyzed by XRD. It detected K2CO3 and metallic K as the most abundant species in every char sample. The presence of metallic K suggested the occurrence of a redox mechanism involving metallic K and its oxide30
33 during the reforming and gasification. 3.5. Mass Balance with Respect to K. The mass balance was examined on K over the range from the K loading to the reforming/gasification. K contents of the individual K-retaining solids, i.e., K-cedar, K-char-P, K-char-R1, and K-char-R2, were determined for a run with the S/C ratio = 1.10. The results are summarized in Table 2. It is noted that the pyrolysis of K-cedar released a substantial portion, 30%, of K into the gas phase together with the volatiles. Volatilization of K during the pyrolysis was thus significant, and this is consistent with previous reports.19,23 More importantly, the net uptake of K by the K-char bed of the reformer was no less than the amount of K volatilized during the pyrolysis [compare “E” with “D + (A
B)” in Table 2]. The K-char bed thus captured the volatilized K quantitatively probably with no or little escape of K out of the reformer. It was also confirmed that major portions of K in K-cedar and that in K-char were extractable with water, which showed extractabilities equivalent to those of a CH3SO3H solution (pH 1). K-char-P and K-char-R2 contained water-insoluble K, which was also acid-insoluble. Inherent ash-forming species, such as SiO2 and Al2O3, were contained in K-char with a total content below 1 wt %.27 These species could react with K forming composite oxides, such as K2SiO3

’ AUTHOR INFORMATION Corresponding Author

*Telephone: +81-92-583-7796. Fax: +81-92-583-7793. E-mail: [email protected].

’ ACKNOWLEDGMENT A part of this work was carried out in a R&D program that was financially supported by the Ministry of Environment (MOE), Japan. The authors are also grateful to the Funding Program for the Next Generation World-Leading Researchers (NEXT Program) established by the Japan Society for the Promotion of Science (JSPS). ’ REFERENCES (1) Milne, T.; Evans, R. J. Biomass Gasifier Tars: Their Nature, Formation, and Conversion; National Renewable Energy Laboratory (NREL): Golden, CO, 1998; NREL/TP-570-25357. (2) Han, J.; Kim, H. Renewable Sustainable Energy Rev. 2008, 12, 397–416. (3) Abu El-Rub, Z. Y.; Bramer, E. A.; Brem, G. Ind. Eng. Chem. Res. 2004, 43, 6911–6919. (4) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Biomass Bioenergy 2003, 24, 125–140. (5) Chembukulam, S. K.; Dandge, A. S.; Kovilur, N. L.; Seshagiri, R. K.; Valdyeswaran, R. Ind. Eng. Chem. Res. Dev. 1981, 20, 714–719. (6) Brandt, P.; Larsen, E.; Henriksen, U. Energy Fuels 2000, 14, 816–819. (7) Abu El-Rub, Z.; Bramer, E. A.; Brem, G. Fuel 2008, 87, 2243–2252. (8) Dufour, A.; Celzard, A.; Fierro, V.; Martin, E.; Broust, F.; Zoulalian, A. Appl. Catal., A 2008, 346, 164–173. (9) Hosokai, S.; Kumabe, K.; Ohshita, M.; Norinaga, K.; Li, C.-Z.; Hayashi, J.-i. Fuel 2008, 87, 2914–2922. 207

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