Catalytic Biomass-Derived Tar Decomposition Using Char from the Co

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Catalytic Biomass Derived Tar Decomposition Using Char from the Co-pyrolysis of Coal and Giant Leucaena Wood Biomass Supachita Krerkkaiwan, Suwat Mueangta, Paweena Thammarat, Lucksamee Jaisat, and Prapan Kuchonthara Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502792x • Publication Date (Web): 22 Apr 2015 Downloaded from http://pubs.acs.org on April 26, 2015

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Catalytic Biomass Derived Tar Decomposition Using Char from the Co-pyrolysis of Coal and Giant Leucaena Wood Biomass

Supachita Krerkkaiwan†, Suwat Mueangta‡, Paweena Thammarat‡, Lucksamee Jaisat‡, Prapan Kuchonthara*‡,§

†

The Joint Graduate School of Energy and Environment (JGSEE), King Mongkut's

University of Technology Thonburi, Bangkok 10140, Thailand. ‡

Fuels Research Center, Department of Chemical Technology, Faculty of Science,

Chulalongkorn University, Bangkok 10330, Thailand. §

Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn

University, Bangkok 10330, Thailand. *

Corresponding author. Tel.: +66 2218 7523-5; Fax: +66 2255 5831;

E-mail address: [email protected]

ABSTRACT: In this study, the catalytic effect of chars from the co-pyrolysis of Indonesian coal and biomass (Giant Leucaena wood; LN) on the biomass derived tar decomposition was investigated using a two-stage fixed bed reactor. The catalytic performance of the single coal char (CC), LN biomass char (LNC) and coal/LN biomass blended chars (C/LN) at three different weight ratios on the catalytic tar cracking under nitrogen (N2) and tar steam reforming under a steam/ N2 mixture was studied. The C/LN showed a better catalytic role for tar decomposition than the CC and LNC. The high coal fraction (3:1 (w/w) C:LN) blended char had a good catalytic effect in both tar catalytic cracking and tar steam reforming due to its high surface area and silicate forms of the alkali and alkaline earth metals (AAEM)

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on the char. On the other hand, char with a high biomass ratio (1:3 (w/w) C:LN) also had a good catalyst activity in tar steam reforming because of the high steam activation of the char surface and the AAEM would be in the form of phenolates. Keywords: Co-pyrolysis, char, rice straw, Leucaena leucocepha, pyrolysis, steam gasification 1. INTRODUCTION Biomass gasification is a promising technology to generate synthesis gas, mainly hydrogen (H2) and carbon monoxide (CO), for energy production in power plants as well as petrochemical production. However, tar formation is one of the major problems that occur during biomass gasification. Tar is a complex mixture of condensable hydrocarbons, which includes single to five ring aromatic compounds along with other oxygen-containing hydrocarbons and complex polycyclic aromatic hydrocarbons.1 At a low temperature, tar causes blockage and/or fouling problems in some equipment, such as filters, engines and turbines, resulting in operating interruptions.2 Recently, catalytic tar decomposition using char from the pyrolysis of coal or/and biomass has become of interest because char production and tar reduction can be simultaneously implemented inside the gasifier by controlling the parameters and configuration.3 Char and char-supported catalysts have been used for the decomposition of model tar compounds, such as benzene, naphthalene and phenol,4, 5 as well as for biomass and coal tar.3, 6-10 The catalytic activity of the char is related to its surface structure and the presence of alkali and alkaline earth metallic species (AAEM), such as Na, K, Ca and Mg, over the char surface.9 A number of studies have reported that the char derived from the pyrolysis of biomass can act as a good catalyst for tar elimination because of its large porosity and high amount of AAEM catalytic species. 6-8 However, the yield of biomass char


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was relatively low compared with that derived from coal due to the lower fixed carbon level in biomass. Chars derived from a low-rank coal, such as Brown coal and lignite, were also reported to be good catalysts for the reforming of nascent tar.3, 9, 10 Improvements in the char surface structure, morphology and reactivity have been observed in the co-pyrolysis of coal and biomass.11-14 These enhancements are related to the transfer of OH and H radicals from coal to the biomass and to the catalytic roles of AAEM, especially potassium (K), during the co-pyrolysis.11 Unfortunately, there is no information on the catalytic effect of co-pyrolyzed char on tar decomposition or the application of coal/biomass blended char for tar reduction. Consequently, in this work, the catalytic effect of co-pyrolyzed chars from the obtained co-pyrolysis of Indonesian coal (sub-bituminous) and Giant Leucaena wood (LN) on the biomass derived tar decomposition was investigated using a two-stage fixed bed reactor. The effect of the coal and biomass blending ratio during the char preparation and the effect of steam addition were evaluated, and the catalytic effect of the pyrolyzed-char on tar reduction was discussed in relationship to the char characteristics, as determined by Brunauer-Emmitt-Teller (BET), scanning electron microcopy (SEM) and X-ray fluorescence (XRF) based analyses.

2. MATERIALS AND METHODS 2.1 Biomass Sample and Char Preparation. The Giant Leucaena wood (LN) biomass used in this study was obtained from the PTT Research and Technology Institute, and was then ground, sieved to a 150–250 µm size, dried at 110 oC for at least 6 h and kept in a desiccator before use. Char was prepared from the slow heating pyrolysis of coal, biomass and coal/ LN biomass blends in a typical fixed-bed reactor as previously reported.11 Pure coal char (CC) and biomass char (LNC) were prepared from Indonesian coal (sub-bituminous coal, particle size of 150–250 µm) and LN (particle size of 150–250 µm), respectively. Co3 ACS Paragon Plus Environment

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pyrolyzed chars (C/LN) were prepared from the co-pyrolysis of the same coal and LN at coal: biomass blending (w/w) ratios of 1:3, 1:1 and 3:1, which are designated as C/LN 1:3, C/LN 1:1 and C/LN 3:1, respectively. For the preparation of all the chars, 7 g of original sample was placed inside the quartz reactor (20 mm internal diameter (ID) and 40 cm heating zone length) and then slowly pyrolyzed under a nitrogen (N2) flow rate of 120 cm3 min-1 heating at 27 OC min-1 to at 600 OC and then held at this temperature for 60 min. 2.2 Decomposition of Biomass derived Tar in a Two-Stage Fixed Bed Reactor. The decomposition of biomass derived tar was performed in a two-stage fixed bed reactor (Figure 1) that consisted of an inner tube (9 mm ID and 60 cm length) and an outer tube (19 mm ID and 89 cm length), and was divided into two parts, each with individual furnaces and temperature controllers (Carbolite model MTP 12 and Lenton model LTF 12 for the upper and lower part, respectively). The upper part was defined as the pyrolysis zone and was where the biomass pyrolysis took place and generated the volatiles. The lower part was the tar-char contacting zone where the prepared char or/and inactive alumina ball (inert bed) was located and the biomass derived volatiles from the upper part contacted with the prepared char at this zone. First, 0.5 g of the prepared char or 7.5 g of inert bed were packed into the lower part to give an approximate 2 cm bed height and then N2 was fed into the reactor at flow rates of 80 and 30 mL min-1 for the inner and outer tubes, respectively. The residence time of volatile in the char bed was approximately 0.3 sec. After purging for 1 h, both electric furnaces were heated up to 800 oC for both zones. In the case of tar steam reforming, water at a flow rate of 0.14 µL min-1 was heated at 300 OC. The heated steam was introduced into the lower part of the reactor at a steam:N2 (v/v) ratio of 60:40. Simultaneously, 120 mg of fresh biomass was dropped into the inner tube (upper part). The obtained biomass char remained on the quartz wool filter in the middle of the inner tube while only the biomass volatiles passed into the 4 ACS Paragon Plus Environment

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lower part. In the lower part, the biomass volatiles make contact with the char allowing the catalytic tar cracking to take place. The mass ratio of biomass feeding and char bed was kept constant at 1:4.2 for all experiments. Some of the heavy tars were condensed in an iced-tar trap filled with isopropanol and 6 mm diameter-round glass beads to enhance its capability for recovering the condensable compounds. The non-condensed (gaseous) products were then collected in a 2 L gas bag for quantitative analysis. The gas collection bag was changed every 10 min during the 2 h reaction time. The gaseous products generated from the char in the lower part without biomass feeding was evaluated in the same manner and used as the reference data.

2.3 Characterization of the Chars, Products and Data Analysis. The proximate and ultimate analyses of the substrates (LN and coal) and their derived chars (LNC, CC and C/LN blends) were performed following ASTM D3172-3175 and using a CHN analyzer (LECO CHN-2000), respectively. Moreover, the specific surface area, pore volume and pore size of the chars were measured by N2 adsorption at -196 OC using the BET method (model Quantachrome, Autosorb-1, instrument accuracy ± 0.11%). Samples were first degassed at 300 OC for 6 h prior to the N2 adsorption. The morphology of the chars was characterized by SEM (model JEOL, JSM-5410LV). The elemental analysis and the composition of solid crystalline of the chars were also performed by X-ray fluorescence (XRF, BRUKER model S8Tiger) and X-ray diffraction (XRD, BRUKER model D8 Advance) techniques, respectively. The X-ray diffractometer employed the CuKα radiation (λ = 1.5406 Å) and an X-ray power of 40 kV, 40 mA. The produced gas, which was mainly comprised of H2, CO, methane (CH4) and carbon dioxide (CO2), was quantitatively analyzed by gas chromatography (GC) using a Shimadzu GC-2014 instrument with a thermal conductivity detector (TCD) and Unibeads C


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column (3.00 mm ID × 200 cm length). 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 from biomass and , represents the mole of carbon in biomass. The carbon conversion into char was obtained from the carbon content of biomass char remained in the inner tube at upper stage by CHN analyzer. Consequently, the carbon conversion into tar was calculated following Eq. (2), Carbon conversion into tar (%) =100 − − where and

!

!

(2),

represents the carbon conversion into gas and char, respectively.

Some of the condensable tar in the ice-tar trap was analyzed to determine its chemical composition using GC-mass spectrometry (MS) on a Shimadzu Model QP2010 equipped with a DB-5ms capillary column (0.25 mm-OD × 0.25 mm film thickness, 30 m length, J & W Scientific) and with helium as the carrier gas. The molecular weight scan range was 30– 500 m/z with a 3.5 min solvent cut time. The column was held at 40 °C for 5 min, and then the temperature was increased to 200 °C at rate of 10 °C/min and held for 25 min. 3. RESULTS AND DISCUSSION 3.1 Characterization of LN, Coal and Prepared Chars. Table 1 summarizes the proximate and element analysis of the Indonesian coal and LN substrates, whilst Table 2 shows the physical properties of the prepared CC, LCN and C/LN blended chars. All these prepared chars except for LCN had markedly lower hydrogen/carbon (H/C) and oxygen/carbon (O/C) molar ratios than the original coal and biomass samples. This was due to the release of volatile matter during the slow pyrolysis of the coal or/and biomass at the high applied temperature (600 oC), which resulted in char formation via secondary reactions, such as polymerization and thermal cracking of the heavier volatile products.11 The LNC had 6 ACS Paragon Plus Environment

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a higher oxygen content than the CC and C/LN blends, except for C/LN1:3, and also had the highest K content but the lowest Si content (Table 2). The AAEM species on the char surface can form phenolates (i.e. K-O-C), in which the metal is chemically bonded with an oxygen atom on the carbon matrix of the char, and silicates (i.e. K2SiO3, Ca2SiO4 and Mg2SiO4), in which the metal is strongly bonded with the silica content of the char. 15,16 Thus, it was likely that in the case of LNC, the AAEM, such as K, Ca and Mg, would be presented as phenolates rather than silicates due to the relatively high oxygen and AAEM content. In contrast, the AAEM in case of CC might largely exist as silicates because of the large Si content. This could be confirmed by XRD pattern of the prepared chars as shown in Figure 3. In case of CC, the peaks of SiO2, K2SiO5 and Ca3SiO7 solid crystalline were obviously presented, while those peaks gradually disappeared for LNC and even the C/LN blended char with a high LN ratio. Unfortunately, the AAEM in a phenolate form was not detected due to the limitation of XRD technique. The different physical properties of the CC and LNC could be expected to result in proportional changes in properties of the different C/LN chars. In comparison with the predicted properties of the C/LN blends (cal. data in Table 2.), a C/LN 1:3 had a significant higher oxygen content, while the C/LN 3:1 and C/LN 1:1 chars had the lower oxygen content than expected. In addition, the Si contents of all C/LN chars were higher but AAEM content (K, Ca and Mg) was lower than the predicted data. It was expected that AAEM would be in the silicates rather than in the phenolates except for the C/LN 1:3 which had the high potential to form the phenolates due to the relatively high oxygen content in the char matrix. This was evidenced by XRD pattern of the C/LN 3:1 which exhibits the crystalline peaks of CaSiO5, K2Si2O5 and Ca2Si2O7 (as can be seen in Figure 3). The phenolate and silicate groups on the char surface have previously been reported to be catalytic in the carbon steam gasification.15, 16 However, these different chemical forms (bonds) of the AAEM with char 7 ACS Paragon Plus Environment

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have different stabilities that relate to the different catalytic activities of each char on the tar decomposition. From the BET analysis, only the C/LN 3:1 had a higher BET surface area and pore volume than the predicted values (21.2% and 7.9%, respectively), whereas the C/LN 1:3 and 1:1 chars had a lower BET surface area and pore volume than predicted (Table 2). In a previous co-pyrolysis study11, the synergy between coal and biomass in terms of char decomposition and char reactivity was less dominant at higher biomass blends. The excess amount of volatiles generated from the biomass probably diminished the reactivity of the blended char, which might relate to the char structure and morphology.11 The enhanced char surface properties of the C/LN 3:1 blend was supported by the SEM analysis (Figure 2), where the CC had somewhat indistinct shapeless particles and a smooth surface of hydrocarbons, while fibrous particles and large pores were observed in the LNC. The morphology of the different C/LN chars clearly appeared as a distinct phase between the CC and LNC, especially in the case of the C/LN 1:1 and 1:3 blended chars. Interestingly, the C/LN 3:1 char had a more porous and homogenous structure like that of the CC and LNC. Thus, the C/LN char blends, and especially the C/LN 3:1 one, would be expected to be good catalysts for tar decomposition, and so were evaluated. 3.2 Effect of Co-pyrolyzed Char (C/LN) on Biomass Carbon Conversion. The effect of the co-pyrolyzed char on the carbon conversion level of LN, both in the tar cracking and tar steam reforming reactions, is illustrated in Figure 4. Note that the reported data are the net amount of gas after deducing the reference value from the total amount of gas obtained with the presence of biomass and char. In the presence of the inert bed (inactive alumina), the carbon conversion level into gas was about 48% and 52% for tar cracking and tar steam reforming, respectively. Thus, the addition of steam did not play an important role in the decomposition of the LN derived tar at 800 OC without a catalyst. In all cases, in the presence 8 ACS Paragon Plus Environment

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of the respective char catalyst, a higher level of carbon conversion into gas and a lower level of carbon conversion into tar was found than in the presence of an inert bed in both tar cracking and tar steam reforming, although this was only marginally higher for the C/LN 1:3 in tar thermal cracking. In addition, the level of carbon conversion into gas during tar steam reforming was markedly increased compared with that of tar cracking. This result inferred that the prepared char can act as a catalyst in the tar thermal decomposition and played an influential catalytic role in the tar steam reforming. These results are in good agreement with previous studies

3, 7, 17

that have also shown a catalytic role of hot bed char (both coal and

biomass chars) on tar reduction.3,7,18. GC-MS patterns of LN derived tar after steam reforming at 800 oC with and without the presence of char (i.e. CC) are shown in Figure 5. It consists of benzene derivatives (toluene, styrene), phenolic compounds (phenol, methyl phenol), 2-rings aromatic hydrocarbons (naphthalene, methyl naphthalene, biphenyl) and polyaromatic hydrocarbons (acenaphthylene, fluorene, anthracene). It was found that both cases gave quite similar results, although the presence of char likely exhibited lower relative intensities of some compounds such as toluene, naphthalene, acenaphthylene and anthracene. This indicates that the tar was reduced but its composition was not significantly changed with the presence of char. Compared with in the presence of CC or LNC in tar cracking, the C/LN 1:3 and C/LN 1:1 chars gave a slightly lower level of carbon conversion into gas, but this was more than 16.5% higher in the presence of C/LN 3:1 (Figure 4). In addition, for tar steam reforming, the presence of C/LN 1:3 and, especially, C/LN 3:1 gave a higher level of carbon conversion into gas than in the presence of LNC or CC, but that with C/LN 1:1 was only higher than that for LNC. The C/LN 3:1 yielded a 22.2% and 37.1% higher level of carbon conversion into gas than in the presence of CC or LNC, respectively. Thus, the char from co-pyrolysis of coal and LN (i.e. the C/LN blends) had a better catalytic activity in tar steam reforming than the chars 9 ACS Paragon Plus Environment

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prepared from the pyrolysis of coal (CC) or LN biomass (LNC) alone. In addition, it was found that the level of carbon conversion into gas in the tar steam reforming in the presence of C/LN 1:3, 1:1 and 3:1 were around 65.9%, 30.7% and 31.5% higher, respectively, than that in tar thermal cracking. Thus, the addition of steam had a significant effect on the catalytic performance of the C/LN chars, especially for the C/LN 1:3 char. The difference in the catalytic performance of the different C/LN blend chars was evaluated by characterization of the spent co-pyrolyzed chars by BET, SEM and XRF analyses. Table 3 summarizes the BET surface area and XRF elemental analysis of the spent C/LN chars. It was found that the BET surface area of C/LN 1:1 decreased from 203.10 to 185.60 and 413.84 to 395.76 m2 g-1 for tar cracking and tar steam reforming, respectively. On the contrary, the BET surface area of C/LN 1:3 and 3:1 after contacting with tar increased both for thermal cracking and tar steam reforming, in particular in case of C/LN 1:3. This could be confirmed by SEM analysis of the spent C/LN chars as shown in Figure 6. It revealed that the spent C/LN 1:1 both after tar cracking (Figure 6(c)) and tar steam reforming (Figure 6(d)) had a dense carbon surface that was covered by tar deposition. On the other hand, the spent C/LN 1:3 and C/LN 3:1 chars appeared to have a porous carbon structure. The SEM image corresponds to the change in BET surface area of the spent chars. The increased BET surface area and porosity of the spent C/LN 1:3 after tar steam reforming was more noticeable than that of C/LN 3:1, whereas the catalytic role of the C/LN 3:1 was more dominant than the C/LN 1:3 as mentioned above. It was probably related to the existence of AAEM chemical forming over the char. Analysis of the AAEM and Si contents on the spent C/LN chars by XRF revealed that almost all the spent chars had lower AAEM contents (Table 3) than that of the corresponding fresh char (Table 2), except for the spent C/LN 3:1 one that had K and Ca contents of 2–14% and 14–58% higher than that the fresh one after tar catalytic cracking and tar steam 10 ACS Paragon Plus Environment

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reforming, respectively. This can be explained by the high amount of Si in the C/LN 3:1 char (Table 2), which probably induced the volatized AAEM, especially K, from the biomass volatiles to generate silicate compounds over the C/LN 3:1 char surface that then acted as the catalytic sites for tar decomposition. Potassium silicate has previously been reported to be effective as an additive for carbon activation to increase the porosity of carbon materials.18 It is, therefore, assumed that the AAEM-silicates on the C/LN 3:1 surface might act as another factor to increase the BET surface area of the spent C/LN 3:1 char, resulting in the increased catalytic activity for tar decomposition. On the other hand, the AAEM on the C/LN 1:3 surface presumably bonded with the carbon of the char matrix as phenolates due to the lower Si content and higher O content, as mentioned above. Phenolates have been reported to be less stable than silicates and so can be easily attached by the excess level of hydrogen radicals from the volatiles, resulting in the loss of AAEM by volatilization, as expressed in Eq. (3); CM − M + H ↔ CM − H + M ,

(3)

where CM, H and M represent the char matrix, hydrogen radical released from the volatile and AAEM, respectively.15, 19, 20 In addition, the amount of AAEM on the spent C/LN 1:3 surface was higher than in the other chars but its catalytic performance was a little lower than the C/LN 3:1 char, which is due to the reactive form of the AAEM and the inhibition effect of the hydrogen radicals derived from the volatiles. However, the catalytic activity of the C/LN chars also reflected the composition of gas product, as described in the next section.

3.3 Effect of Co-pyrolyzed Char (C/LN) on Gas Product Distribution. The effect of the co-pyrolyzed chars on the gas product distribution obtained from tar cracking and tar steam reforming of LN is shown in Figure 7. With respect to the tar cracking, in the presence of inert bed (no char), the main gaseous products were CO and H2, which were generated from the decomposition of cellulose and hemicelluloses from LN at the high applied


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temperature.21 In the presence of almost all the chars, higher CO and H2 yields were obtained because of the catalytic tar decomposition over the char. The exception was with the C/LN 1:3 char where a lower H2 and slightly lower CO yield was obtained. During tar catalytic cracking, the auto-generated steam produced from the biomass feedstock can react with hydrocarbons via tar and carbon steam gasification, as expressed in Eqs. (3) and (4), respectively:3, 11, 17, 21, 22

[C x H y O z ] + H 2 O ↔ CO + H 2 ,

(4)

C + H 2 O ↔ CO + H 2 .

(5)

With respect to the tar steam reforming, the addition of steam resulted in markedly higher H2, CO and CO2 yields, and particularly in the presence of the chars. This was attributed to the essential catalytic tar steam reforming and carbon of the char in the presence of the excess external steam, as shown in Eqs. (4) and (5). In addition, the presence of C/LN 1:3 char gave a higher proportion and yield of H2 and CO2 production than with the other chars, which is probably due to the high phenolate content on the C/LN 1:3 surface, as discussed above. In the presence of external steam, the AAEM can form phenolates with the carbon matrix of the char, which favors H2 and CO2 production and promotes the volatilization of AAEM,15 as shown in Eqs. (6)–(8): M + H 2 O ↔ M (O ) + H 2 ,

(6)

M(O) + C ↔ C(O) + M ,

(7)

C (O ) + M (O ) ↔ M + CO 2 ,

(8)

where M, M(O) and C(O) represent the alkali metal, alkali metal bonded with carbon and the intermediate of carbon on the char, respectively. This phenomenon is then assumed to be less


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dominant for the C/LN 3:1 char since the AAEM is likely to be more in the form of silicates than phenolates, and so accounts for the lower H2 and CO2 yields in the presence of the C/LN 1:3 char. Accordingly, the transformation of the AAEM on the C/LN char surface to phenolates or silicates is an important factor that influences the gas production during tar steam reforming.

4. CONCLUSIONS In this study, the catalytic effect of chars obtained from the co-pyrolysis of Indonesian (subbituminous) coal and biomass (Giant Leucaena wood; LN) on the biomass derived tar decomposition was investigated in a two-stage fixed bed reactor. The blended char (C/LN) provided a better catalytic performance in the tar decomposition than either the CC or LNC chars, in terms of having a higher carbon conversion level into gas and higher H2 and CO yields, especially in tar steam reforming. The char surface area and AAEM levels on the C/LN blends were significant factors that influenced the catalytic activity of the char. The high coal blend C/LN 3:1 char gave the best catalytic performance in both tar cracking and tar steam reforming, which is due to its high surface area and the presence of AAEMsilicates. The high biomass blend C/LN 1:3 performed as a good catalyst in tar steam reforming because of the steam activation of the char surface and the formation of AAEMphenolates. These results are beneficial in the design and operation of co-gasification of coal and biomass systems.

ACKNOWLEDGMENTS


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The authors appreciate the financial support from Thailand Research Fund and Fuels Research Center, Department of Chemical Technology, Chulalongkorn University.

REFERENCES (1)

Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G., A review of the primary measures for tar elimination in biomass gasification processes. Biomass Bioenerg. 2003, 24, (2), 125140.

(2)

Abu El-Rub, Z.; Bramer, E. A.; Brem, G., Review of catalysts for tar elimination in Biomass gasification processes. Ind. Eng. Chem. Res. 2004, 43, 6911-6919.

(3)

Sun, Q. S.; Yu, S.; Wang, F. C.; Wang, J., Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of hot char. Fuel 2011, 90, 10411048.

(4)

Burhenne, L.; Aicher, T., Benzene removal over a fixed bed of wood char: The effect of pyrolysis temperature and activation with CO2 on the char reactivity. Fuel Process

Technol. 2014, 127, 140-148. (5)

Fuentes-Cano, D.; Gomez-Barea, A.; Nilsson, S.; Ollero, P., Decomposition kinetics of model tar compounds over chars with different internal structure to model hot tar removal in biomass gasification. Chem. Eng. J. 2013, 228, 1223-1233.

(6)

El-Rub, Z. A.; Bramer, E. A.; Brem, G., Experimental comparison of biomass chars with other catalysts for tar reduction. Fuel 2008, 87, 2243-2252.

(7)

Gilbert, P.; Ryu, C.; Sharifi, V.; Swithenbank, J., Tar reduction in pyrolysis vapours from biomass over a hot char bed. Bioresource Technol. 2009, 100, 6045-6051.

(8)

Brandt, P.; Larsen, E.; Henriksen, U., High tar reduction in a two-stage gasifier.

Energy Fuels 2000, 14, 816-819.


Page 15 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(9)

Zhang, L.; Matsuhara, T.; Kudo, S.; Hayashi, J.; Norinaga, K., Rapid pyrolysis of brown coal in a drop-tube reactor with co-feeding of char as a promoter of in situ tar reforming. Fuel 2013, 112, 681-686.

(10)

Min, Z. H.; Yimsiri, P.; Asadullah, M.; Zhang, S.; Li, C. Z., Catalytic reforming of tar during gasification. Part II. Char as a catalyst or as a catalyst support for tar reforming. Fuel 2011, 90, 2545-2552.

(11)

Krerkkaiwan, S.; Fushimi, C.; Tsutsumi, A.; Kuchonthara, P., Synergetic effect during co-pyrolysis/gasification of biomass and sub-bituminous coal. Fuel Process Technol.

2013, 115, 11-18. (12)

Zhu, W. K.; Song, W. L.; Lin, W. G., Catalytic gasification of char from co-pyrolysis of coal and biomass. Fuel Process Technol. 2008, 89, 890-896.

(13)

Park, D. K.; Kim, S. D.; Lee, S. H.; Lee, J. G., Co-pyrolysis characteristics of sawdust and coal blend in TGA and a fixed bed reactor. Bioresource Technol. 2010, 101, 6151-6156.

(14)

Yuan, S.; Dai, Z. H.; Zhou, Z. J.; Chen, X. L.; Yu, G. S.; Wang, F. C., Rapid copyrolysis of rice straw and a bituminous coal in a high-frequency furnace and gasification of the residual char. Bioresource Technol. 2012, 109, 188-197.

(15)

Okuno, T.; Sonoyama, N.; Hayashi, J.; Li, C. Z.; Sathe, C.; Chiba, T., Primary release of alkali and alkaline earth metallic species during the pyrolysis of pulverized biomass. Energy Fuels 2005, 19, 2164-2171.

(16)

Fushimi, C.; Wada, T.; Tsutsumi, A., Inhibition of steam gasification of biomass char by hydrogen and tar. Biomass Bioenerg 2011, 35, 179-185.

(17)

Krerkkaiwan, S.; Tsutsumi, A.; Kuchonthara, P., Biomass derived tar decomposition over coal char bed. Scienceasia 2013, 39, 511-519.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(18)

Page 16 of 24

Kong, J.; Yue, Q.; Gao, B.; Li, Q.; Wang, Y.; Ngo, H. H.; Guo, W., Porous structure and adsorptive properties of hide waste activated carbons prepared via potassium silicate activation. J. Anal. Appl. Pyrol.2014, 109, 311-314.

(19)

Sonoyama, N.; Okuno, T.; Masek, O.; Hosokai, S.; Li, C. Z.; Hayashi, J., Interparticle desorption and re-adsorption of alkali and alkaline earth metallic species within a bed of pyrolyzing char from pulverized woody biomass. Energy Fuels 2006, 20, 12941297.

(20)

Wu, H.; Quyn, D. M.; Li, C.-Z., Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part III. The importance of the interactions between volatiles and char at high temperature. Fuel 2002, 81, 1033-1039.

(21)

Zhang, Y.; Kajitani, S.; Ashizawa, M.; Oki, Y., Tar destruction and coke formation during rapid pyrolysis and gasification of biomass in a drop-tube furnace. Fuel 2010, 89, 302-309.

(22)

Smolinski, A., Coal char reactivity as a fuel selection criterion for coal-based hydrogen-rich gas production in the process of steam gasification. Energy Convers.

Manage. 2011, 52, 37-45.

Table (s) caption Table 1. Proximate and element analysis of the coal and biomass samples Table 2. Physical properties of the obtained pyrolyzed chars Table 3. BET surface area and XRF element analysis of the spent chars Figure(s) caption


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Energy & Fuels

Figure1. Schematic diagram of the two-stage fixed bed reactor, which consisted of a (1) nitrogen cylinder, (2) mass flow controller, (3) distillated water reservoir, (4) HPLC pump, (5) steam temperature controller, (6) electric furnaces, (7) quartz reactor, (8) sample feeder, (9) iced-tar trap, (10) bubble flow meter and (11) moisture trap (filled with silica gel).

Figure 2. Representative SEM images (1,500 x magnification) of the (a) pure coal char (CC), (b) pure biomass char (LNC), (c) C/LN 1:3, (d) C/LN 1:1 and (e) C/LN 3:1 blended chars, all prepared from slow pyrolysis at 600 OC. Scale bars represent 10 µm.

Figure 3. XRD patterns of the prepared chars Figure 4. Effect of co-pyrolyzed char on carbon conversion both in tar thermal cracking and tar steam reforming of Giant Leucaena wood. Data are shown as the mean + 1 SD, and are derived from 3 independent repeats.

Figure 5. GC-MS patterns of Giant Leucaena wood derived tar after steam reforming at 800 O

C with and without the presence of char.

Figure 6. Representative SEM images (1,500 x magnification) of the (a) spent C/LN 1:3 after thermal cracking, (b) spent C/LN 1:3 after steam reforming, (c) spent C/LN 1:1 after thermal cracking, (d) spent C/LN 1:1 after steam reforming, (e) spent C/LN 3:1 after thermal cracking and (f) spent C/LN 3:1 after steam reforming.

Figure 7. Effect of co-pyrolyzed chars on gas product distribution from tar thermal cracking and tar steam reforming of Giant Leucaena wood. Data are shown as the mean + 1 SD, and are derived from 3 independent repeats.


Energy & Fuels

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Table 1. Indonesian coal

Leucaena leucocepha (LN)

Proximate analysis (wt%) Moisture

12.41

8.89

Ash

8.39

2.59

Volatile matter

36.84

62.21

Fixed carbon

42.36

26.31

Ultimate analysis (daf wt%) Carbon (C)

72.13

48.39

Hydrogen (H)

6.67

7.11

Nitrogen (N

1.40

0.29

Sulphur (S)a

0.22

0.14

19.58

44.07

H/C molar ratio

1.11

1.76

O/C molar ratio

0.20

0.68

Potassium (K)

0.11

0.62

Sodium (Na)

0.03

0.02

Calcium (Ca)

0.69

0.483

Magnesium (Mg)

0.15

0.13

Silicon (Si)

3.18

0.30

Oxygen (O)

b

Elemental content (wt%)c

a

Determined by the Bomb washing method (ASTM 3177) Determined by difference c Characterized by XRF analysis b

ACS Paragon Plus Environment

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Energy & Fuels

Table 2. Char sample

LNC

C/LN 1:3 Exp.a

C/LN1:1

Cal.b

Exp.

Cal.

C/LN3:1 Exp.

CC

Cal.

Ultimate analysis (wt%, daf) 77.02

72.37

78.91

81.68

80.80

84.23

82.69

84.58

Hydrogen (H)

1.81

1.99

2.08

2.57

2.34

2.13

2.61

2.87

Nitrogen (N)

0.99

1.02

1.08

1.27

1.17

1.21

1.26

1.35

Oxygena (O)

20.17

24.62

17.93

14.47

15.69

12.43

13.44

11.20

H/C molar ratio

0.28

0.33

0.31

0.38

0.35

0.30

0.38

0.41

O/C molar ratio

0.20

0.26

0.18

0.13

0.15

0.11

0.13

0.10

Potassium (K)

1.81

1.01

1.45

0.60

1.09

0.35

0.73

0.37

Calcium (Ca)

1.27

0.90

1.50

0.67

1.74

0.49

1.97

2.21

Magnesium (Mg)

0.19

0.18

0.18

0.13

0.18

0.13

0.17

0.17

Silicon (Si)

0.44

2.59

1.19

2.19

1.93

3.33

2.68

3.42

276.3

248.4

252.2

208.6

228.2

248.0

204.1

180.0

Pore volume (m g )

0.256

0.223

0.228

0.162

0.201

0.1862

0.173

0.145

Pore size (AO)

37.12

35.97

35.90

31.07

34.67

30.02

33.45

32.22

Carbon (C)

Element analysis (wt%)

BETSA (m2 g-1) 3

-1

a

Exp. refers to the experimental data

b

Cal. refers to the calculated data


Energy & Fuels

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Table 3. Char sample

C/LN 1:3

C/LN1:1

C/LN3:1

After heating without tar

250.82

203.10

213.90

After tar thermal cracking

297.40

185.60

278.80

After heating and steam contacting without tar

383.15

413.84

286.85

After tar steam reforming

489.9

395.76

328.06

 Potassium (K)

0.92

0.38

0.36

 Calcium (Ca)

0.85

0.58

0.51

 Magnesium (Mg)

0.19

0.16

0.14

 Silicon (Si)

2.68

1.24

3.89

 Potassium (K)

0.99

0.58

0.40

 Calcium (Ca)

0.99

0.76

0.60

 Magnesium (Mg)

0.23

0.22

0.20

 Silicon (Si)

2.66

2.57

4.23

BETSA (m2 g-1)

Element analysis (wt%) After tar thermal cracking

After tar steam reforming

Figure1.


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Energy & Fuels

(a)

(c)

(b)

(d)

(e)

Figure 2.

Figure 3.


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Carbon conversion (% of carbon in biomass feed)

Energy & Fuels

Page 22 of 24

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%

CHAR TAR GAS

Inert LNC C/LN C/LN C/LN CC 1:3 1:1 3:1


Tar steam reforming

Tar cracking

Figure 4.

Figure 5.


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Energy & Fuels

(a)

(b)

(c)

(d)

(e)

(f)

Figure 6.


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Gas production (mmol/g of biomass feed)

Energy & Fuels

Page 24 of 24

CO2

60

CH4 CO

50

H2 40 30 20 10 0 Inert LNC C/LN C/LN C/LN CC 1:3 1:1 3:1


Tar steam reforming

Tar cracking Figure 7.


Catalytic Biomass-Derived Tar Decomposition Using Char from the Co

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